Actually you "can" make Windows XP Pro utilize 4 Gb Ram:

http://www.gehrytechnologies.com/catia/catia/catia_windows_large_memory.htm

http://www.microsoft.com/whdc/system/platform/server/PAE/PAEmem.mspx

http://support.microsoft.com/kb/291988

However, I think simply adding another Gb for a total of 3 Gb would make a big difference.  The only reason I went with 4Gb is that I found it cheaper to buy a matched pair (Corsair) than it was to buy a single module to add to the 2Gb I already had.  

And BTW:  the above info is for XP Pro NOT for XP Home.