Windows Vista Upgrade or Not?

I currently have Windows XP and I am thinking about upgrading to Windows Vista for Directx 10. So I have a few questions on Vista:

1) If I upgrade to Windows Vista can I go back to Windows XP without having to backup my data?

2) Can I use the same serial number for 2 of my computers? (Is it legal?)

3) Are my system specs (as listed below) high enough to have Windows Vista running quickly?
Asus P5GC-MX/1333 MOBO
EVGA NVIDIA GeForce 8600GT 1GB DDR2 Video Card
1GB DDR2 667 Trancend Ram
Intel Core 2 Duo 1.6ghz Processer
450W Power Suply
(Sorry I forgot to post them earlier)

Thats all I have to ask. Please reply quickly because I only have 2 weeks to decide! Thanks!

1. No, but you could set up a dual boot.
2. No
3. Don't know, probably. Without a CPU, amount of RAM, and video card used its hard to say for sure. Will it install, yes, will it run as nice as XP? Depends.

In my opinion vista is not worth the problems it has atm. I would simply stick with what works (xp) until they get vista sorted out.