i see that everyone is crazy about your ram and cpu problems, but no one answered your HD question (As far as i read)
if you were to put another HD in just for windows, you might as well go w/ a smaller cheap one, that would then become your primary HD (since you wanna run and OS off of it). Your current settings on watever OS will not transfer over (unless you do that one thing XP has for that, migration tool or watever), it'll be like a brand new OS. to make more space on your 80 gigs, uninstall (if you know how to) windows to drop another 1.5Gigs or so of space.
now, if you really wanna be a baller and ****, you should go ahead and buy one of them i-RAM harddrives, put like 3GB of RAM in it, and then it'll boot and run CRAZY fast, no joke. but it's expensive (no duh).