Okay... for anyone who thinks you can happily tool away in XP with 4Gb or more of RAM, I suggest you check out the following website:
Roll On Linux!
This quote in particular is not kidding:
Although the performance benefits of 64-bit are somewhat dubious on the desktop, a 64-bit OS absolutely essential if you run applications that need to use more than 2 GB of memory.
(Ungrammatical, but informative. Also note that this problem is still apparent even in 32-bit Vista. Yikes!)
I've been there, I've had to troubleshoot RAM problems using 4Gb of RAM in XP Pro for a friend, and although I conceed that the PC in question was a little extreme (A greedy SLI setup with twin 512Mb video cards didn't help any...) the damn beast only showed 2.5 Gb of RAM in XP when there was 4Gb installed and recognised by the BIOS.
It all boils down to the more hardware in your system, the less RAM you have to play with. Especially if you have PCI-Express hardware.
4Gb is great for Linux(64-bit compiled, of course) and 64-bit XP or Vista... but realistically overkill for anything else.
This is my last comment on the subject in any case. But for further research, might I suggest you type "windows xp memory limit" into Google...
GrouchoC.