That is the lowest you want to go, but you should be fine if it is a name brand PSU. If it isn't, then I might upgrade.
If you plan to upgrade to 2 GPU's then you will need a more powerful PSU.
It really depends on the brand. Name brand, like Thermaltake, Cooler master, pc power and cooling, zalemen, or something along those lines should be fine. If it's an off brand, I would suggest investing a little bit of money into a good, brand name 750ish watt unit. That will be more than enough for this build, and you will more than likely be able to use it for most if not all future builds, as they tend to last a long time.
Okay, thanks for the replies, but can anyone answer my second question? "And what happens if you don't have a power supply with a high enough wattage?"
Either, right out of the gate, the PC won't boot up, or will shut off when trying to pull a higher wattage.
Really, with that build (granted this is a known brand PSU) 550W should be fine. I used to run a two GPU, two HDD setup 5 years or so ago on a 600W OCZ PSU and was completely fine.
What is the brand of your current PSU? That Might help us gauge better if it should be reliable enough to use or not
At work we have a tester, and a lot of the non-name brand PSU's come up short on what is written on the side of them.
If you can afford it, I'd get a new 750w supply. As others said higher up, 550 is just about okay, but it would be a good idea to upgrade. Worst case scenario, is that when you are running a game, the machine tried to draw too much power and the machine will turn off, and this could potentially damage your components.
Power supplies degrade over time and with that their output is reduced.
GW is likely to be a Great Wall, it's not the best in PSUs. Having said that a couple OCZ and Thermaltake PSUs used to be rebranded Great Wall PSUs. They weren't great and were prone to catching fire but again that would most likely be cause people assumed having a brand name meant having a quality PSU and overloaded their units. When looking at getting a new PSU best thing to do is look up reviews, there are duds with all manufacturers pretty much and it's best to just choose wisely after doing some proper research. If you want to be lazy about it, rather than checking the brand name on the box, check the brand name of the OEM who built the unit.
I'd recommend getting a new PSU along with the rest of your hardware. As for which one... depends on exactly what hardware you have. We could estimate and depending on whatever the rest of your hardware is (2-4 RAM modules, 1-2 SSD/HDDs, Case/HSF Fans, Fan controller etc), your PC at load would likely use around 300-400W, so I'd say go with a 600W minimum. I'd recommend going with a 700W+ though as considering your motherboard, it leaves you with the option of adding another card to run SLI for any extra performance in the future.
EDIT: Forgot to mention decent OEMs - Seasonic is probably one of the best and they're also a group who have their own retail products. They make PSUs for a couple of the major brands (Antec, Corsair, XFX). Next would be Enhance (Antec, CoolerMaster, Silverstone, Zalman) and CWT (Channel Well Technology) who many would might be familiar with since they produced the Corsair HX-750, HX-850, HX-1000, HX-1050, all the GS series and many of the CX series as well as about half of the TX series. If the PSU is made by one of these OEMs then you're likely to be pretty safe.
I made a bunch of assumptions, but this site suggests around 370 W for your setup. 550 W is fine and 750 W is way overkill. You might consider getting a better brand of PSU, though.
Nvidia recommends a minimum of 450 W, which sounds about right. I usually go with the GPU supplier's recommendation since that will be the most power-hungry part of your machine.