I'd also recommend multicore as well. Definitely a major difference for me going from a single core box to a dual core box for my main workstation.
I'd go for quad core if you're seriously interested in things like isolating things in VMs as far as dedicated computing hardwaare. A 4proc is *lots* of room to expand, and enough that even if you are giving VMs multiple virtual processors, say 2 per VM, one core getting pegged won't cause the kind of negative performance spiral you would get if you were having 2-proc VMs on a 2-way box (in such a case I would just go for single proc VMs).
For desktop stuff, quad core is likely to be much less noticible than dual core in the general case. Again, I've noticed a significant gain in terms of keeping large numbers of VMs running smoothingly with 4-way over 2-way, but that is probably not something you would be running on, say, a gaming machine.