Ok, there are two main types of processor, obviosley, AMD & Intel.
Seeing as how you have previosley used Intel, may I suggest another Intel..? Definetly dual core, you will love it.
Clock speed - It's the standard, factory speed of a processor, or anything for that matter. 2.0Ghz seems to be standard now, although, some people want more, so alot of processors are being produced that hit the 3.0Ghz mark. Obviosley, buying a faster one, means a faster computer, but remember, you could always overclock, if you really wanted to.
FSB - (Front Side Bus) - It denotes the speed at which your processor interacts with the components on your motherboard, such as RAM, etc. FSB speed is hitting the 1066Mhz mark now, but tpyically, they are 333Mhz, 600Mhz, and 800Mhz. Then, clock multiplyer comes in..overclockers manipulate the stock speeds, ie 500Mhz FSB X 5.0 clock multipler = 2500Mhz = 2.5Ghz.