I'm using a 7800GT.
I was playing Crysis yesterday at 1280x800, Textures/Objects/Shadows/Physics/Post Processing on Medium, and everything else on Low. I am also using a custom autoexec.cfg that actually does seem to work by providing a performance increase with little loss in visuals. And it actually runs well enough to where I can increase a few details. Problem is that Shaders at Low really hurts quality, but with a shader-weak card like the 7800GT, it kills performance even at Medium.
I play COD4 at 1680x1050 with most all details on low or off, but since I use SM2.0 now, I can up other details. Source games, BF2, Quake Wars, Joint Operations, all those run fine at native res and high details. So really I can play games that are a little bit dated at native 1680x1050 and usually max details.
Crysis, RB6 Vegas 1/2, and STALKER are demanding though. Dynamic lighting is a huge performance hit in STALKER (shader weak 7800GT again). But static lighting is very fast, so I use the static lighting and 1680x1050 with mid details instead of 1024x768 with dynamic. RB6 Vegas is just as demanding as Crysis I think, and the first one came out in 2006!