Single socket quad core is the best bang for buck.
If you REALLY need more power, a dual socked quad like skulltrail will work, but they are MUCHOS PESOS!!!
Xeon chips arent any faster, but they are theoretically better 'quality' chips. They should last longer, oc better, and run cooler. Obviously you pay extra for that. However, the current consumer chips are SO good, most people dont bother with xeons.