To help with Vista: I've had it for almost 3 years, and only had one BSOD. Most of the freezes and uncalled shutdowns in Windows is the user doing something stupid, anyways.
And for 7: I had the Beta(Although I got the RC, but never installed it because I only have two sufficient hard drives, which I'm currently using and pack with 100+ GB of data), and it was AMAZING. Now, it's really really hard to upgrade from XP to 7, but I'd say it's totally worth it.
HOWEVER. You should wait until next year, maybe, or SP1, to be sure you will have minimal trouble since you said XP is working fine for you.
And read the system requirements... that's what people forgot to do with Vista, and look what happened to them.