My Opinion==
Dont invest on XP now, get a Windows Vista Beta DVD (or download it), & install on your PC.. as its BETA its FREE & Valid till JUNE 1, 2007.
& when the final version is released you can buy it.
& as you are going for a Processor & M/B upgrade (i think), wait few more months, coz when Intel's QUAD Core Processor is released, the Dual Core Prices SHOULD come down. so at that time you can get the Dual Cores at cheaper rates.