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.