Well the first and obvious answer from us is….NONE!  You don’t need it, shouldn’t upgrade your existing computer to it and certainly should not buy a brand new PC with it.  (All you can get in the retails stores is Vista, but as a Dell Value Added Reseller, we can still order new PC’s with XP until June 30, 2008!)
But if you are just bent on getting Vista, here is something to help…
The above link will take you to a site with a nice explanation and matrix to help you decided.  If you have trouble accessing it, just let us know and we’ll send a document to you for your review.