I have never seen such a resource pig as Vista. You'll need a fairly powerful system--not the $400-500 specials or you WILL pull your hair out. Also, make sure you know what software you NEED to run, because there are still a lot of compatibility problems. Research the software on the internet first.
I bought a new laptop recently with a dual core AMD chip and 2 GB RAM. It is BARELY passable. I plan on waiting 6 more months before I start to think about updating my old box. When I do, I'll probably look at assembling one myself and put in 4 GB of memory and a quad-core processor.
Bryan