Well ones you can play 3d games with (atleast UT2k4), like the graphics cards in some laptops, that have a PCIe connector.. I bring it up, because it would be possible to build better small PCs, if we had graphic card options.
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jd_ is correct, HOWEVER (and this is a big one), small cases are often not great at dealing with a lot of heat. Even with fans, it is very easy for hot spots to develop. Unless it exhausts its heat (like the HIS Black Hole / IceQ Turbo coolers do), I'm not sure I'd put anything that gets hotter than a HD7770 in a small case.
I had a $150 mobo fry a VRM shorted in a small case. It was stock and maybe slightly undervolted, but the HD7870 I had in there produced a lot of heat during games, and one day it simply died. The case was a Lian Li PC-Q08R, which has two fans providing a flow of air through the case, but it was apparently not enough. Lesson learned.