Based on my experience and what I've read, Vista doesn't really use multiple cores any better than XP.
It all depends on the applications that are used, and whether they are written to take advantage of multiple cores.
Alternatively, if several applications are run at once, each can use a different core.
I don't think the OS is really a factor unless it doesn't support multiple cores/CPUs, which all modern operating systems do.
boo, I would actually recommend the quad core if it's not much more expensive than the E8200. I have the E8200 myself and it's a great performer, but I have found myself wishing I had a quad core at times. The only reason I didn't get a quad was because it was out of stock and I needed the computer ASAP.