1. XP will not recognize more than 3 gigs of RAM (3.5 in some cases)regardless if it is DDR2 or DDR memory.
2. 2 Gigs of RAM for XP is fine.
3. Only the 64-bit version of Vista (can't say for the XP 64 bit version can because very few run it and drivers are a pain to find) will utilize a full four gigs (or more).
4. Not even Vista 32-bit versions will utilize 4 gigs because like XP it is a 32-bit OS and they are limited to the amount of RAM it will recognize.
If you want to know what type of RAM your system can take and how much I suggest going to
www.crucial.com and allow their online scanner to scan your system. This is totally safe and the only thing they need to do is dowload an Active X in order to scan. I have done this on many systems and you will not get malware/adware.