Are you sure this is relavant to Windows 7?
First a general guideline:
If you use it for desktop applications (which I doubt in your case), you'll need a CPU that handles many active running processes at once and does better in multitasking, so I'd suggest the QuadCore.
If you want to optimize
background services, though, Xeon has the lead on that. That's why its mostly used on server machines.
Now on your case, I believe QuadCore will serve you better.