If at all a 32 bit OS limits anything then it is the memory count.. Core count has nothing to do with it.. As an OS though, newer versions like win 7 can make more effective use of a multi core processor compared to XP.. Anyway, there is not much of a price difference between an Athlon II X3 and X4.. So getting the X4 is a smarter decision.. And on a side note, please look forward towards moving to win 7 64 bit..
Yes, the x4 is better, but what are his uses? For gaming, you would benefit from a X4 more. I use an X3 and love it though. Works great and I game with COD Black Ops, SC2, and BFBC2 at max and get great frames. Also, Win. 7 is better, but really, he doesn't need to spend a lot of money on it. XP is still a good OS.