Since you are not a techno nerd stay away from linux. I am a techno nerd, so linux is an option. Also, vista is more trouble than it's worth. XP will work just fine. I was in your situation, just needed a new cpu. I went to
www.visionman.com and orderd a gaming desktop and was extremely happy with my purchase.
I could have gotten a bare bones and built it with new parts, but I was lazy and wanted it 80% built. The parts I'd have needed to move from my old desktop were pretty old anyway.