He's going to need DDR2 for that motherboard, not DDR.
If you are running Vista, a full 4GB can help with your performance. But if you're running XP, 2GB is about the sweet spot for gaming.
Your best bet would be 2x 2GB DDR2 800 PC2-6400 memory such as these: OCZ 4GB (2 x 2GB) 240-Pin DDR2 800
if you go that route.