The main thread of a Windows program is the UI thread. If, on the main thread, you stop to count to a billion, the whole program will become unresponsive, moving another window over the program will leave it a white hole on the desktop. You spawn other threads from the main UI thread to do the background work, leaving the UI responsive to input.
I would probably only see a real speedup with an 8 core if I wasn't connecting to a remote DB.
Now I need a few Windows VMs to test. A while ago in another job I had a need for four simultaneous virtual machines on my system. Running that on a one-core machine is NOT fun. I'm thinking of writing a program that will need eight to properly test. I will not do that unless I have a Mac Pro.
In your case, the testing network is out there and actually exists on other hardware. In my case the network is on my development machine. The question is what's cheaper? Do we buy moderate boxes for development, LDAP server, database server, file server, web front-ends, etc., with the space and electricity they require? Or do we just buy one really fast development machine and run all that virtual? Six VMs means each gets a core and your dev box keeps two for itself. 8 gigabytes means each gets a gig (well, LDAP can run in much less) and your dev environment gets over two.