If you need the RAM, get it. I really don't think the performance drop (if any) will be noticable and it will, in the long run, be offset by the added memory. I suggest you use a 64 bit OS though. While i can't speak for you, having installed 4GB and having only, say 3.2 usable by the machine, would bother me to no avail.
I agree, I plan on using vista home premium or ultimate next year on one drive and xp pro on the main drive, since i plan on getting a phenomx4(or q6600, if the phenoms suck to much) and either a 4850 or 8800gt, I want to have everthing covered so i can play my old games and the new ones that are coming out. Plus i wanna do a litting video editing change my vhs to dvd's. Im babbling, but i hope you guys understand what im talking about.