Vista to windows seven?

a little while ago i bought an emachines W5233 computer so i could be some what up to date. the only problem was that it came with windows vista which slowed the mid-low range ( now mid-range with 8500) considderably. this problem was promptly solved a week later when i put another harddrive in and installed XP on it. my SATA vista hdd is just sitting in there collecting dust and i am thinking of formating it for the sake of higher bandwidth. the only problem with that is there is some data on that disk that i would like to keep thats not too important. im anticipating on getting windows 7 when it comes out because it is supposed to run as fast as xp with all the prettyness and dx10 support as vista. so the question is should i format my SATA drive and use it for xp or do the upgrade thing with vista when windows 7 comes out. thx