Core 2. I have to say that I don't actually use the desktop much - one box just serves my intranet CVS repository/web and holds a RAID 1 .. and the other one pretty-much only runs Xine. Both boxes seem fast enough running the desktop, and they're not all that powerful (server is a dual Celeron 466 with 384MB). I found that the accelerated NVidia drivers helped the multimedia machine's performance quite a bit.
If I'm wanting to use a desktop for anything then it's Windows for me, anyway. The only reason the multimedia machine isn't running a Windows variant is that the DXR3 card is unsupported in Windows XP and the Linux drivers are more stable and useful.
