Dual core processors are 2 cores on 1 die...they share memory though...whereas a dual processor setup gives each CPU it's own ram....which in your case might be beneficial.
If you decide to go with a dual core setup you will save a lot of money and have more options.
If you go with a dual CPU setup it will be more expensive and you'll have less options...
For Dual core I suggest going with Conroe...it's Intel's newest line of CPUs...very efficient and very fast. They support 800mhz DDR-2 and most if not all motherboards that run them support 8gb ram.
If you go for a dual CPU setup then it depends how much you want to spend. If you go AMD you will have to get a socket 940 board which only accepts socket 940 opterons which are pretty expensive. But there are dual CPU AMD motherboards that support dual core cpus so you can have a total of 4 cores!
Or...you can go with intel..but that way is pretty expensive too...you will need Xeon processors which are pretty good..but kinda expensive and AMD keeps up with them very well