he'll be able to handle at least 800x600 quite smoothly. i have a slower 1.1ghz athlon runnin 1024x768 smoothly wit a gf2 gts with graphics turned down. a gf4 ti4200 and a 1.2 should be able to handle it with some eye candy.
Athlon XP 1.58GHz | MSI KT3 Ultra2 | 256MB DDR PC-3000 | GeForce 4 ti4200 64mb @ 310/533 | IBM 120GXP 40.0GB | Det. 41.09 | DX 9 RC2 | Win XP SP1