As I said in my very first PC building story, I think 8GB is the sweet spot for RAM right now, across AMD and Intel platforms. To quote that post:
"RAM is cheap these days. 8GB is plenty for gaming, and an 8GB kit of DDR3/1600 costs around $40. There’s no need to cut corners farther than that. Get a kit with two 4GB sticks, so you can utilize dual-channel mode on your motherboard. You’ll be able to add another 2 x 4GB kit later, or even a 2 x 8GB kit, as you’ll be getting a motherboard with four RAM slots. That’ll let you upgrade to 24GB of RAM before you even consider tossing the RAM you have."
I could stop this guide right there, but it bears fleshing out a little bit. So we'll take it step by step: first we'll talk about how much RAM you need, then what speed. We'll dip into multi-channel mode, speeds and timings, and making sure your RAM works well with your system. Don't worry--we'll talk about the just-announced Haswell chipset from Intel, too.
Eight Is Enough (For Now)
Why 8GB? I'd consider 4GB the rock-bottom, barrel-scraping minimum amount of RAM necessary to run a modern computer--say, an ultrabook--in such a way that it doesn't usually feel like it's getting hung up on everyday tasks. Even then, I'd try my damnedest to get more RAM. But if you're building a desktop, it's likely because you want more computing power than you get from an ultrabook--either for gaming or some other high-performance task. 8GB is enough for even very intensive games, though video editors, programmers, and people who do a lot of memory-intensive work will want more RAM. 8GB is a good starting point, and you can build from there if you find it isn't enough.
Just by way of comparison, here's my PC using 4GB of RAM while only running CrashPlan, Dropbox, Rdio, and about 15 Chrome tabs (including several Google Docs tabs). Imagine if that's all the RAM you had to work with.
For most general-purpose and gaming rigs, 8GB is the price/performance sweet spot, and this is likely to remain the case with the next generation of AMD and Intel chipsets as well.
Interestingly, DDR3 prices have gone up in the past few months: A good 8GB DDR3/1600 kit now costs around $60, rather than the $40 it cost when I wrote my first column in December. Acer's JT Wang says it's because DRAM factories are prioritizing smartphone DRAM over desktop DRAM.
Dual Channel Mode: or Why Two Sticks Are Better Than One
Nearly all modern consumer motherboard chipsets have dual-channel memory architecture (X58 has three channels, and X79 has four). Two channels means twice the total memory bandwidth. Not twice the performance, mind you--just twice the total available bandwidth of single-channel architecture, which helps remove bottlenecks. So does this actually make a difference? Well, it can't hurt. Tom's Hardware showed single-digit improvements in gaming performance while using dual-channel architecture (way back in 2007), while Maximum PC saw huge gains in integrated graphics performance when switching a laptop from single- to dual-channel modes.
If you have a motherboard with four RAM slots (pretty much everything you can get today except Sandy Bridge-E mobos and some mini-ITX boards) and you do decide to get 16GB instead of 8GB, go for two 8GB DIMMs instead of four 4GB DIMMs. Why? Because if you actually need that much RAM, you're likely to want more later. Two 8GB sticks will leave two empty slots in your motherboard that you can populate with two more 8GB sticks later on, whereas if you had gotten four 4GB DIMMs, you'd have to remove some of them in order to put in higher-capacity DIMMs.
Oh, and if you're wondering how to enable dual-channel mode on your motherboard: just put the RAM into like-colored slots (this has been the standard for years, but consult your particular motherboard's reference guide in case you're unsure).
Timing Is Everything: How Fast, How Furious?
What clock speed should you aim for? That depends on your motherboard and chipset. Ivy Bridge and Haswell do very well with 1600MHz DDR3, but can benefit from higher-clocked RAM.
AMD's Trinity platform with its integrated GPU scales up very well with higher RAM clocks--you should get at least DDR3/1866, and Tom's Hardware found excellent performance scaling up to DDR3/2133.
In Anand's tests, Ivy Bridge performance returns diminish quickly after 1866MHz RAM.
AnandTech recently did a big comparison of Ivy Bridge RAM performance using similar G.Skill RAM clocked from 1333MHz to 2400MHz, and found a decent performance increase when you go from 1333MHz to 1600MHz, and another one when you move from 1600MHz to 1866MHz. Returns diminish quickly after 1866MHz.
Regarding Haswell, Intel's newest desktop architecture: per Ars Technica's Haswell introduction, "1600MHz DDR3 is Haswell's highest officially supported speed, same as Ivy Bridge." That doesn't mean higher-clocked RAM won't work; Anand's Haswell piece mentioned that Haswell should work very well with high-clocked memory and included this tantalizing line: "Ivy Bridge was pretty decent at higher speeds, but Haswell is a different beast entirely." We should see in-depth memory tests from Anand and others soon. Your motherboard's manual should also tell you the maximum supported memory speeds on your specific mobo.
What To Look For When You're Buying
Once you've narrowed down your RAM search by capacity and speed, you're left with...way too many options. Want two 4GB sticks of desktop DDR3/1600? Newegg has over one hundred different kits. So how do you narrow them down?
The QVL: Generally, any RAM with the right voltages, timings, and capacity that fits in your motherboard will work--in that it will function as RAM without blowing up your computer--but there are always weird exceptions, and you definitely want to make sure it'll run at its rated speeds with your system. Each motherboard manufacturer publishes a Qualified Vendor List, or QVL, for each motherboard, on their support page. Here's the QVL for my motherboard. If you want to be absolutely sure the RAM you buy will work with your motherboard, get something from the QVL. At the very least you should check your motherboard's recommended voltages, timings, and capacity, even if you get RAM that's not on the QVL.
Manufacturer: RAM is increasingly commoditized, and there are many manufacturers with solid reputations. Crucial, Corsair, G.Skill, Patriot, Kingston, and Mushkin all make good RAM. With so many good RAM makers there's no reason to look elsewhere.
Those fancy fins of aluminum are mostly for show, like the plumage on a tropical bird.
Plumage: Sorry, y'all. Those fancy fins of aluminum are mostly for show, like the plumage on a tropical bird. Stock-clocked DDR3 just doesn't get very hot, and even the moderately-tall heatspreaders that have become the norm (see Corsair's Vengeance series) can interfere with large CPU heatsinks, which DO have a function, but tend to overhang one or more RAM slots. I'm not going to pretend I've never chosen a specific RAM kit just because the heatspreaders looked good with the rest of my build, but that should be a very minor consideration--CPU cooling comes first.
Overclocking Shmoverclocking: Overclocking your RAM and manually setting the timings can let you squeeze more performance out of your memory, but my official policy is not to bother. The effort is not worth the end result, which will be barely noticeable. It's not like GPU or CPU overclocking, which is pretty easy and offers more obvious performance increases.
Memory Profiles: That said, many RAM kits come with XMP profiles, which let the motherboard read certain optimized presets and set the RAM speed and timings appropriately--often for higher speeds than the kit is supposedly rated for. Those are easy enough to set and revert without the risk of breaking things, and usually they're the only way to make sure that that kit of DDR3/2133 is actually running at its rated speed, rather than whatever the BIOS decides to set it at. Intel platforms use XMP; AMD has a memory profile system called AMP.
Crowdsourcing: Check the reviews. If a kit has 1,400 five-egg reviews, it's probably a good buy. Three stars or below? Avoid. Again, there are so many different RAM kits available that you should be able to find a good kit at a competitive price with good ratings. There's no sense settling for less.
The Future Is Coming: DDR3 or DDR4?
DDR3 has been out for nearly six years, but does that mean you should hold out for DDR4? Nope! DDR4 is years from coming to desktops, so your DDR3 should be able to migrate to your next build--and maybe the one after that.
For general computing and gaming, 8GB of RAM is the sweet spot. Get a set of two 4GB DDR3 DIMMs, at at least 1600MHz. If you're running AMD's Trinity platform, you should get DDR3/1866, but DDR3/2133 is a good step up if you can afford it. If you're on Ivy Bridge or Haswell, DDR3/1866 is a good step up. Don't worry about fancy heatspreaders or the lack thereof, unless you have a big CPU heatsink. Get RAM from a reputable manufacturer, preferably something from your motherboard's Qualified Vendor List, and make sure it's set in the BIOS to run at its rated speeds.