I used to build all my own PCs for many years. These days I find it's more cost effective to buy a ready-built Dell. I can get a complete machine for typically less money than the bits I need to buy to do an upgrade. Dells aren't the most rip-snorting machines, but they do everything perfectly adequately well, offering very good value for money.
Have a look on the factory outlet site for base unit only deals, as you presumably don't need monitor/KB/mouse. There's some bargains to be had if you look hard enough.
http://www1.euro.dell.com/content/de...=uk&l=en&s=dfo
Vista is much better since SP1 came out, haven't had any issues with it.