I would agree just hold off a bit, and upgrade all of it.
The only real advantage the quad will have is with multi-threaded apps.
If you do alot of such work, it would be a slight upgrade. but beyond that about the same.
Essentialy, it'll be just as fast but with 2 extra cores.
Thing is though, for the price, you could easily upgrade to newer arcitechtures which is faster clock for clock, with each core being faster.
Even single thereaded apps would see a improvement if moving to a new tech.
Even if if you do a lot of multi-threaded work, it still may not be enough of a upgrade to justify the cost on the current platform.