Am I the only person who uses Vista day in and day out and has not had a single problem with it? Granted, I have a decent spec laptop (1.8 GHz C2D and 3 GB RAM) but that is not even remotely uncommon today! And I bought mine around 13 months ago. The only problem I've had was with Zone Alarm which kept knocking out my internet connection, but once uninstalled I've had no trouble! I run it for months without shutting down, use sleep and hibernate modes all the time AND run quite a few programs at the same time (including mathematical analysis software).
I think that Vista SP1 is mature now. Get yourself a laptop and an OEM disk instead of the junk companies that make laptops put on them. And these days many companies are doing a "buy it with Vista today and upgrade to 7 for free" offers, you may want to look into those.
Either way, get a late edition dual core and 3 or more GB of RAM, and as long as you know the software you use runs on the OS you're getting, it's fine. XP won't be patched for too long, with Vista/7 you're guaranteed patches and fixes for quite a few years more.