In that case I would stick with the 5500 Xeon chipset as they're faster and energy efficient as well. See if you can get the higher end Processors (X5550+) where you can have up to 16 cores on a Dual Processor setup with Hyper threading enabled.
When selling Virtual Servers, both CPU and RAM are important. If you have the budget I would recommend upgrading the RAM to 16GB as well, RAM is cheap these days.
Actually the 5410 and the 5500 series are rated at 80W, unless you're ready to pay extra for the low voltage 5500 series. From the 5500 series I'd recommend at least getting 5520 since it has HT, while the 5506 does not.
In a server environment you won't really notice DDR3 speed improvements over DDR2.
High Bandwidth Servers
Custom Hosting Solutions
A dual 5570 server supports almost twice as many VMs as a dual 5470 server, despite running at a lower frequency.
From the benchmark:
"The newest Xeon is shattering records again"
"it can consolidate by far the most VMs in a dual socket server."
"It is already dangerously close to the quad socket servers"
"World switch times from VM to hypervisor have been reduced to 40% of those of Clovertown (Xeon 53xx), and EPT is good for a 27% performance increase."
"There is little doubt that the newest Xeon is also the champion in virtualization."
One thing about 5506 though is that it lacks hyperthreading. I'd go with 5518 or 5520 at least, which support hyperthreading and offer double the amount of logical cores. That should really help virtualization performance.