You will need 1 gig of RAM to get it running the way you want it. For the money you'll spend, your really just getting a cosmetic upgrade, nothing substancial that makes it worth $200. Personally I see Vista as the ME of XP. The reviews have been mediocre to down right savage. Honestly if you want to invest in another OS, experiment with Knoppix. You can download it and run it off a CD, so no worrys on installing it on your hard drive if you do not like it. And with Beryl graphics it will blow any and everything away that windows has ever offered Plus its Free!
For the newbie just getting into non-MS operating systems, and who doesn't want to take the Mac plunge just now, I'd recommend Linspire - it's a Linux-based Windows-workalike - in other words, the user interface is very much like Windows, so the novice can use it.