I run the same configuration (q6600 8gb Vista ultimate 64bit)
All 4 cores are used by vmware. Each individual VM is limited to a max of 2 cores usage. I run 4 VM's and when they are all busy, you can see the total cpu of all 4 cores approach 100%.
I have a 2900 with dual E5320 CPUs. VMWare uses all 8 CPUs, as mentioned though you are limited to 2 CPUs in the guest, have to make the jump to ESX to make use of more in guests.