How many CPU charts do you have in your task manager?
How many CPU's are showing in your Device manager?
A dual core uses both cores. If it is not, then something is wrong.
Now about the disabled core thing and making it sound way over simple.....
The latest generation AMD cpus are quad cores. To get the new triple core CPUs, they use the same die, they just disable 1 core. The disabled core could be from a perfectly good quad, or they may have been defective in some way so they would not work correctly as a quad. Some motherboards will allow you to force the disabled cores to work. You might get lucky and get one that works, or you might not. It is a crap shoot.
Want more performance? If your board supports it, buy 1 of the new quad, or tri cores, (phenom II's) and clock them at about 3.4ghz.
That should do you nicely. If you want more than that, you will have to go i7.