FlatOut 2 runs very smoothly on my old Athlon XP 2500+, 512 DDR and GeForce FX 5700 with full detail at 1024x768. And I mentioned it because it's a great game, as well ;) Also, Half-Life 2 and Episode One are very tolerant with older systems. Oh, and I almost forgot Call of Duty 2, wich has a DX7 setting that's very useful for older rigs.