Will it really make a difference if I upgrade to a Quad-Core?

Because I currently have the following processor:

Intel Pentium Dual-Core @ 2.7GHz

As you can tell... not the best. I don't game very much at all, but I do a lot of HD rendering, game development, Photoshopping, and things like that.

Whenever I render, it goes up to about 89% and I can't really use any other program well at all. I upgraded my motherboard and everything, I went from 4GB of DDR2 to 8GB of DDR3.

I'm thinking about getting the Intel Core 2 Quad @ 2.66GHz. It's roughly at about $190 or so. The clock speeds are about the same, but will the extra cores really make a difference? How does multicore work exactly, does a program just use one core or will it try to utilize all of them? Could one core do the rendering while other cores do other stuff? I don't want to waste my money.

Extra question, what programs other than games use your GPU? I use Sony Vegas and it can get pretty slow with HD videos, I have the NVIDIA GeForce GTX 450... it should be running well, I'd say.

Look at my signature for further information on the specs, thanks.