I'd give Vista a miss if you're planning on this system being your main one. It's still not really stable enough for that for my liking.
If it was a for a 2nd system for playing around on, then go for it, but if you really need it to working without any problems 99% of the time, stick with XP untill after the first Vista Service Pack comes out.
I've just finished building a new system, and set it up with 2 seperate installs. 1 of XP, 1 of Vista. The XP install is fine, hasn't crashed once in over 3 weeks now. Vista is falling over every other hour.
All the hardware is 'Vista Certified' and came with Vista drivers, and I downloaded updated versions of them where they were available and did all the available Vista updates as well. Still falls over every other hour or so.
Doesn't bother me, since it's only on so I can start learning the OS and where things are, what the common faults are, etc... so I can provide support to customers, but not what I'd call suitable for a working system.