I'd go for Vista 64-bit... Might as well get 8GB in there

Get the Q9300, or Q9450 if your budget allows.. the 45nm processors run cool and use less power than the slightly older 65nm ones.

If you're storing your files locally on the computer, might not be a bad idea to have to disks in RAID0 (striped) for the performance increase that gives you. (get like two 250's or whatever you want)..