Best GPU using one card slot?

HD5770 is two-slot.
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
HD4870 with one-slot, but it the larger fan on it might use another slot.

HD5670 doesn't use two slots if you want to crossfire those.

I'd got for the HD5670. It's pretty much the best single-slot solution out there now. Add in the fact it doesn't need a dedicated power connection and it's a sealed deal
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

ON the nVidia front, I'd go for a 9800GT. It's performance is on par with the 5670,
though, you'll end up trading Eyefinity and DX11 for PhysX.
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Single slot and doesn't require additional power connector. There's a couple like it, just have to make sure you look.

+1 to the 5670. You might be able to find single-slot versions of a few higher end cards, but they're gonna be pretty expensive.

If you can find one of the older 4850s those had one slot coolers on them.

Thanks everyone; 5670 it is.

I guess somewhat ironically, I had the 5670 that tikrjee linked to in my saved cart at newegg, I just wanted to make sure I wasn't going to miss out on anything better.

Thanks again everyone.

I win!  

I guess you win, too! Though, your prize is awesomer

The HD5670 does have its advantages but the 9800GT is a decent amount more powerful;
http://www.techpowerup.com/reviews/HIS/Radeon_HD_5670_I...
They are about the same price so it's a judgment call on your part; about 10% performance vs DX11

Damn it, I said "I Win!"
j/k. Yea, the 9800GT is more powerful. But it also draws more power, so there is that trade-off to consider too. More power=More heat
The 5670 does also bring Eyefinity (though, with this card, I'd limit it to office application usage), not to mention the whole slew of Dolby TrueHD and DTS-HD support for when you want to do more than game. Of course, there's DX11 (honestly, though, with the games that support ATM, it's not that big a deal until better driver support gives it a boost), and more of those titles are coming  
Though, you do get PhysX support with the 9800GT, which I feel is an OK trade-off with DX11 (buyer beware: 9800GT does NOT support DX10.1, only DX10)
But the Home Theater potential and prospect of less-demanding DX11 titles in the future seal the deal for me

Those cards may only have 1 PCI expansion slot taken up from the outside, but the coolers on (almost) all of the stick out far enough to block the adjacent slot, thus effectively making it a dual-slot card.

http://www1.sapphiretech.com/us/products/products_overv...
I have owned this card and it has not given me any problems in terms of it taking up more than one slot. Right below it was a airlink awlh6080 wireless internet card if it matters.

Said "almost" all of them. I know there's a couple exceptions. Heck, some of the cheat and send the bulk to the backside of the card (crafty, no?)

There's also a single slot GTX 260;
http://www.pcper.com/article.php?aid=816

Wow-- that is lot to squeeze into one slot-- I suppose they made up for it in length  

I would have checked that one out a bit more had I not already ordered the 5670, but really, I game little, hence the need fo the trade off-- i could get a sweet video card I will rarely use but I will use 8 processors and all my pci cards everyday, well not everyday, but I will do a lot more decryption than gaming.

Thanks all-- I can in all good conscience pick a best answer when there was more than one solid answer to my question, but I will say tikrjee won