I would recommend that you focus on getting the slowest components of your system upgraded. For example, the secondary storage (hard drives) are really what makes your computer slow. You can spend top dollar on CPUs but if you have slow performing storage, you are going to get the value out of the CPU.
You can only be as fast as your slowest component.
biggest difference is dual= 2 cores and quad =4 cores ,then more cores the better according to the makers of processors ,the more cores the more commands the cpu can handles at any one time .
google.com will have more acurate info that mine of coarse good luck
A lot of systems advertize dual core cpus with hyperthreading as quad-core, which is deceptive in my opinion. My work laptop is an i7 cpu, which should be quad-core for real, but a lot of them are only dual-core + hyperthreading, and such is my system. That means I can't run as many cpu-intensive operations as I should be able to, which can be a problem given that I run Windows 7 as the host OS, and Linux as a guest in a virtual machine, and I can only allocate 1 core to Linux, even though I could well use a couple of more!
From what I've been able to figure out where I work from our reps is that the more cores available to the computer the more it can delegate the programs currently being run to allow for more efficient processing. If you have a quadcore processor you can crunch more numbers faster for multitasking because of the extra cores.
Not necessarily more expensive in regards to quad vs dual. There are quite a few quads for under $120 and most duals (the good ones anyways) still run over $80 so in its honestly better just to jump for the quad if you're building your own machine. If you're buying a retail model your better off going with a dual most of the time regarding the price being higher for a decent quad with similar specs.