As far as I know, the problem is not that XP will not 'support' 4GB, as far as I'm aware, it actually will, it's just that it won't allocate more than 2GB to a single process, or at least that's the case with the 32Bit version, the 64Bit version is capable of doing so. Some older Asus motherboards can cause issues too apparently, with a bottleneck at one of the bridges. Even so, it is still worth getting 4GB of RAM, just don't expect 4GB of work on a single app.
Chock