I guess it all depends on the game. Before upgrading my Vid Card I had a Voodoo 5500. Great for its time, but not DirectX9 compatible. Wouldn't run crap. I upgraded to play NFS Underground.
My CPU Really slows me down with games like DOOM3. I have set all kinds of stuff really low to keep it playable.
I'll check out some of the other games people have sugested, but not to hijack the thread... Is there any good Flight/Fighter Sims that aren't so damn hard to play that they're no fun, out there?
I had a real good one called US Navy Fighters back in the 90's, but it runs in a DOS environment. Not quite XP Friendly. Was never able to get it to work again with the newer hardware.