I noticed some conversations from 2009 and April 2010 on Low Profile Graphics cards. I think the HD5570 (1 gb) was the last one semi-agreed upon. Anything newer? I have a 460 watt power supply and a small (but not slim) case and only one 16x slot. The computer has an I7 processor with 12GB memory.
The GTS 450 and the HD 5750 suggested by rolli59 seem like nice cards, but apparently they run hotter and noisier and use more power (not a problem for you) than the HD 5570 and although they only need a single PCIe slot to connect to they are fatter than a "normal" card so they take up the space over the next slot along the motherboard. They are also more expensive - all this for an extra few frames per second in some game.
If your case can accomodate one (you said "small but not slimline") then maybe you should consider a full height card - without the compromises ussually associated with low profile cards. A Core i7 with 450W PSU and 12G is a pretty high speck machine - a low profile card will be the weakest link in such a system.