Another couple of options if I go with 800 RAM would be:
FSB, CPU X, CPU Freq, Mem X, Mem Freq
350, 9, 3.15, 2.4, 840
400, 8, 3.20, 2.0, 800
Not sure I can run 4 GB with XP. Somewhere I think I read XP can only handle 3 GB, unless that was changed in SP-3.