Which Phenom II for Virtualization on AM2 mobo

Could anybody tell something about direct relation between number of cores (X2 vs. X4) and it's influence on speed for virtualized systems as VMware, and ESX, and Hyper-V while using Athlon x62 X2 and Phenom II X2/ X3 /X4?

I have three AM2 mobos ASUS M2N32 Workstation Professional running because of their two PCI-X 64Bit-/ 133 MHz Slots with AMD Athlon 64 X2 6400+ BE on it - not overclocked yet - for getting experiences in virtualization and clustering for simulating High Availability and think about upgrading CPUs to Phenom II X4 - one out of these three: 945 or 955 (best choice?) or 965 - to stronger the host systems running in virtualization mode.

There are 8 GB Corsair DHX Twin2x2GB DDR2-6400 CL4 4-4-4-12 2T or DDR2-8500 CL5 5-5-5-18 2T on every board. Cooler is Skythe Orochi SCORC-1000. PSU is Corsair TX750.

I know I could take X4 940 BE because somebody tried it and found out that it works fine with BIOS V1801 instead of last BIOS V2001. But I want to be prepared and saving money with the CPUs if I have to change to AM3 mobos later what could be next step of investment.

Do you know any reason why I shouldn't take 9x5 instead of 9x0 and what the maximum for x (2, 4, 6) have to be? It's clear that HT (2000/1600) is the bottleneck of these mobo but don't care about this because can't substitute mobos at the moment. Any knowledge and/ or experiences with this kind of upgrading an use of virtualization?

All ideas and/or suggestions are wellcome.

get the low power versoin of the 945