Depends on your needs and budget. I'm not sure that the IDE of VS by itself would require a quad-core CPU, but the testing of your application may get a performance boost with a quad-core.

I would keep your old PC close to test the performance of the application on a less powerful PC.

>Would I see any performance gains if I exceeded that? If I bought a quad-core with 4GB, would I just be wasting my money on unused capacity?