Sorry I interpreted your comment to be regarding the i5 dual to i7 dual upgrade, which is very marginal.
I'm a believer in buying computers for our needs now. I think for most people that's the right thing to do because in general software lags behind. When I read the OPs activities and I look around at family and friends that are doing similar things with their computers, their money would mostly be wasted by upgrading to quad core CPUs.
There are many tasks that will never scale greatly with additional cores. The only way these things get better is by heavy multitasking, and average folks simply won't do that. The question isn't whether they are better to have.. simply whether or not they are a wise way to spend money.