with that much data it's stupid to be spreading it over multiple single drives, at least setup a server w/ a raid 5 or 6 array. it's no replacement for real backups but will protect against drive failures.
XP isn't that bad, and hell it's 7 yrs old now....
Vista got a bad rap due to crappy early driver support and despite ending up as a pretty solid OS was never able to change that early perception.
Windows 7 has been very good so far.