@fisk0: I know there are single slot 6770 models, and I think there is a single slot 6850 model. Do you need single slot because you have another add-in card (like a sound card) that a dual slot card would take up the space for, because if not, a dual slot card should work just fine as well.
What other specs does your machine have? Without knowing that, it's hard to give a good idea of what card would be well matched with your machine.
AFAIK (and you're right, trying to actually find any single-slot GPU in stock anywhere is a pain, even news reports of which ones are offered by manufacturers - many of which end up with their official site either listing discontinued or not mentioning at all that model) nVidia partners didn't really explore the 500 series (at least the ones with enough power to game well on) as a single-slot solution so you're probably looking at the 460 ones you've already seen. The 460 is a tad old and wasn't exactly setting the world on fire at launch but they apparently dropped in price to the point where they weren't a bad deal.
On the AMD side then there is hope. As got some single-slot love. They dropped the performance a bit to make sure it didn't produce too much heat (to the point where I wouldn't expect it to be any faster than the 460 in a head-to-head so availability, price, and your love for either company's software guys* are probably going to be the big factors deciding what you get) but PowerColour came to the plate and even bothered to release the 'Dirt3 free inside' promotional editions which means this was more than a paper-launch thing that they later pretended never happened. Considering that the 6850 is a $140 card, the only places I could find claiming stock to sell were taking the piss. I have a horrible feeling that this being such a niche product means you'll be looking high and low to find someone who isn't just planning to rip you off unfortunately.mentions, the 6850
On the issue of AMD CPU/chipset with nVidia graphics, I haven't heard anyone shouting about either party cutting off their nose to spite their face (I think AMD GPUs connected to AMD mobo/CPUs does automatically overclock the interlink but that isn't the limiting factor in gaming performance so real world I don't think it actually gives you much of a benefit and they may not even do that any more - any automatic overclock should not be able to increase performance by any more than you manually setting an overclock would provide). A quick Google of your motherboard model number or the chipset number combined with 'nVidia' might be due diligence but I'm not expecting you to find anything about incompatibly issues.
* Driver design, messed up game launches, value added random extra features, working harder to make AA work on all the games you want to play rather than accepting that some games can't have AA (I don't care if it halves my frame-rate, one day in the eventual future I'll have enough GPU power to play it at the refresh rate of my screen with no frame-rate drops with that option switched on so give me the option to switch it on or force it on in the driver).
Edit: completed a sentence I forgot to end
Thanks! Seems like the 6850 or the 460 are the best options right now then, unless I want to wait and see if anything turns up with the new cards in a few months.
Yeah, it seems overly expensive considering the performance of the cards, but hey, I decided to go for an m-ATX board and gotta live with that decision I guess. :)
To help you on this, here is a good round up of best single slot graphics cards for gaming and work. You can check this at https://graphicscardhub.com/single-slot-graphics-card/
One of my personal favorites is the GALAX GeForce GTX 1070 KATANA, but it's a little pricey though. Works amazingly and you can run your games mostly in ultra settings.
Feel free to check the review out from here: