Some things you need to know about virtualisation, from the ground up:
:- There are two types of virtualisation - Hypervisor (Type 0) and Type 1. With type 0 you run the hypervisor "server OS" in the lowest mode of the processor (ring 0), then you run client operating systems within the hypervisor. Very useful for servers, doesn't work for gaming as the clients don't have access to the full power of the graphics cards.
Type 1 is what you're looking at, having something like XP or Windows 7 installed and running VMWare or MS Virtual PC / Server, and then installing operating systems inside them.
For Type 1, on 32-bit Intel (x86) you can use *any* processor, however the Virtual Machines will be slow, as Intel Architecture is crap for virtualisation. For 32-bit clients on x64 Windows, same story.
For 64-bit clients on 64-bit it's a bit different. To do this you need a processor which has Intel VT (or AMD's equivalent) instructions. Not all Intel 64-bit CPUs have this, although both the Q9550 and i7 do. With VT the main hardware bottleneck with a Virtual Machine becomes graphics, and in some cases, disk read-write speeds (as the VMWare has to translate the clients OS r/w calls into the host OS's calls, etc).
Also, VT provides a massive performance boost CPU wise as instructions pretty much become 1-for-1.
Relating to your original question, over the q9550 the i7 will only provide marginal gains, as like I said, the VM will rarely use 100% CPU, and both procs have the VT instructions. However, the Westmere architecture (which I've been told will fit in a Socket 1366 motherboard) is supposed to bring more performance boosts to the VT instructions, so you'll have to wait and see.
