You don't really need a server OS for personal use unless you need to predominantly run background services. Even then, you could configure XP to operate in a server-like manner (although you obviously won't have the same degree of administrative control).
To do this:
Under system properties (right-click my computer), advanced tab, under performance go to settings. Advanced tab again, and select the options to adjust for best performance of background services and system cache (under memory).
This will significantly increase the amount of physical RAM available, however applications might take a bit longer to start up.