The only thing with Quad core is that it's kind of new and I'm not sure how they compare to dual core yet. Logically, quad core is "suppose" to be better because it has "four" cores which equals to better multitasking right? In that sense, it makes me think that a quad core CPU is better than dual core. I could be watching a movie, encoding some sound, burning a CD, and even have a game running in the background all at the same time with no significant slow down (pending RAM and other config of course).
But on the other hand you are spending a lot, I mean A LOT of money on a CPU and I'm sure you want to have the very best. Maybe the X6800 is better because of the high clock speed, but the Kentsfield core is quad core which is also pretty good too. Hmmmm. Maybe make another thread with a poll on which CPU to get. See what other people would like. Either way, the Intel Extreme CPUs are really nice to have--dual core or quad core.