Theres a very large thread like this on the rage3d forums, so go there for loads of ideas.
Anyway, here are a few games/programs that look good.
RalliSport Challenge,
demo available. Looks amazing.
Unreal2,
demo coming out soon, dont go too crazy with the FSAA/AF, the game runs slow on any system.
3D Mark 2003,
downloadable, will seriously stress your system, looks amazing, just dont expect fps over 100fps in all tests, ok
ATI Tech demos.
Goto ATI website, theres loads of tech demos there. big downloads, so its not for 56k users(like me, dam). You need to DX9, and Catalyst3.0 or above.
Hope this helps.