I were buying a new PC right now, I'd definately buy one with Vista Home Premium preinstalled. That's because I'd expect to get several years of life (say, 3-to-4) out of a new computer and Vista is where it's gonna be at for the majority of that time period.
On the flip side, I had a PC running XP now, I don't think I'd bother upgrading. I'd just wait till I replace the entire computer.
But then again, take my opinion with a grain of salt; I've never actually owned a PC, and I've never used Windows.
okemyeye