It depends what you do.
For example, I like to game, and play newer games, so i got the PII x4 955. Works great for WHAT I USE IT FOR.
Though if your just surfin' the net and what not, playing slightly lower end games, a dual core would be fine.
This thing is strictly an HTPC. It's hooked up to a tv and I watch DVDs and record tv with it. I was using it to encode video files to various formats but noticed some stuttering/slow response while watching video in Vista Media Center. So, I'll probably stop doin that and use the quad core setup for that purpose too.