Well, P4 CPU that you mention will overclock well, so you will end with a more powerfull PC. As for DDR, I will go for RDRAM 1066 if you can, is the best for hungry bandwith P4.
If you don't plan to overclock and are more budget restricted, go for the XP2100+ and DDR333.
Anyway, both systems can hold ANY games, because the critical factor here is the graphic card. Suggestion: save some bugs with the XP config and spend those $ in a ATI 9700 Pro. Your games are gonna fly ;-)
DIY: read, buy, test, learn, reward yourself!