I forgot to ask about your budget. How much can you spend?
Also you don't really need to get a low profile card, you can get a decent sized card to fit in there.
Well, my budget for the whole build is $350 (lol), and so far with all parts I've managed to spend...$254. Somehow. eBay and Amazon are wonderful places. I was looking at maybe a 5770 if I can find one for ~$90 (eBay?), because they seem to be pretty solid cards.
It also helps that I already have a monitor, speakers, mouse, and keyboard. I guess eventually I'll need an optical drive, but I probably have one of those kicking around somewhere as well.