W7 or XP. Vista is shite.
Today I got to compare - XP, Vista and Windows 7.
XP - as we all know - fast, easy to use, familiar.
Vista - ...Ugh. First, I used a HP PC. HP mouse, HP keyboard, HP everything. Even though the PC had better CPU and more RAM than mine, Vista loaded up in... 5 minutes. Then it bombarded me with list of things I can do before I finally managed to start doing stuff. And then zoink, I couldn't watch any movies from my pendrive in Media Player. I had to go look for my trust-worthy SubEdit, just to find out that it shows no video, just the sound of the movie, even after I downloaded every codec necessary. Only Gom Player managed to play correctly... Also, notifications about downloading AND installing new stuff drove me crazy.
W7 - loads as fast as XP, maybe a bit faster, works great... and if it wasn't for 512MB RAM on my laptop, I would install a copy of W7 there, too. But right now even XP is lagging there, and I'm waiting for my 2GB RAM memory.
Go with W7. Get a copy of Release Candidate quickly, then use it till March and buy the original version when you feel like it's worth. If not, switch to XP and enjoy support till 2014. Vista's support ends in 2012 anyway.