The amount of ram means almost nothing to his performance considering he has 512MB. More important is what mobo/chipset/speed ram does he have. Upgrading to 1GB of PC133 would be a total waste of money. IMO, So would a 6800 ultra/GT as has been recomended. <b> IF </b> he is running PC133, anything beyond a R9800 Pro at most would probably be too limited by system/cpu/mem bandwidth to see improvements. More ram won't hurt things by any means, but no way will a P4 on PC133 be able to play new games like D3 on the highest details anyway. IF it's a DDR board, then that's a different story, but do we know the mobo yet? If so I missed it.
ABIT IS7, P4 2.6C, 1GB Corsair XMS 4000 Pro Series, Radeon 9800 Pro, Santa Cruz, TruePower 430watt