For general multitasking, two cores is all you need (I don't mean running several CPU-intensive programs at the same time, since that is very rare).
Four cores is for the most part only useful for encoding, in other words video/sound editors. On the other hand, the 8800GTX is bottlenecked by a C2D at over 3.5 Ghz, so as long as a game can use the extra cores (for a game programmer that's tough to do) you will see quite a nice boost when running Core 2 Quadro.