Quad core will provide a very limited performance boost as compared to dual core's performance boost over single core, which is still marginal.
The only reason there is any performance boost comes from offloading threads to another CPU. However, very few applications are threaded and even fewer run small enough threads to give any performance boost. Once a thread cannot occupy the local CPU cache, it has to be fetched from memory.
I really do not see the cost/performance benefit of quad core CPU's. I chalk them up to more marketing hype than anything else. Remember this one fact. The only performance gain you can have happens as long as the CPU does not have to access RAM or anyother external I/O.
Only one core can access the outside world at a time. This includes RAM, disk, video card, USB, and the like.