Just remember that 32 bit versions of XP and Vista will not see any ram past around 3.4 GB. If you want to use 4 GB and actually use it all, you have to go 64 bit.
If you're gonna spend $300 just on the cpu, you've gotta have this guy:
http://www.newegg.com/Product/Product.aspx?Item=N82E16819115042