>Visual Studio won't take advantage of the additional cores, but the OS will, so you will see some speed improvement.
>

It looks like you can actually configure Visual Studio (in the 3.5 framework) to take advantage of multiple cores during compilation. It looks like you have to build from the command line to take advantage of it, though.

http://blogs.msdn.com/msbuild/archive/2007/04/26/building-projects-in-parallel.aspx

I can still bring a dual core CPU to it's knees (start compiling and fire up a virtual machine), so I've debated getting a quad core CPU during my next upgrade; that will probably not be for at least another year though.

From the benchmarks I've seen though, if you're having to choose between a slower clocked quad-core CPU vs a higher clockrate dual-core, the higher clock-rate dual core is faster.

I/O is also a real performance killer. Fast drives really make a difference.