Jim,
I built the system I'm using last fall. It consists of a Gigabyte K8U motherboard, two 160 gb Hitachi hard drives, 1 GB of Corsair DDR400 memory, a 256mb video card (Nivida), and an AMD Sempron 3300+ processor. I picked up the motherboard and CPU from TigerDirect as a "combo deal", and got the rest through Newegg.com.
I'm no computer expert, but if I understand correctly, the L1 & L2 cache on the processor, and the memory have the most to do with the speed of a computer. The Sempron 3300+ that I'm running now has only 128mb of L2 cache, where as the Athlon 64 3400+ that I just ordered has 512mb of L2 cache. Most of the motherboard companies have pages on their websites that will show you the "Processor Compatability" for your motherboard. Mine will take up to an AMD 3700+, but those are pricey and hard to find. The processor that I ordered is next in line, and will likely be as high as I'll ever go in this machine.
A really good tool for finding out all the componets in your machine is a program called Everest Home. The company no longer offers it, but you can download a free copy from here.....
http://www.softpedia.com/get/System/...-Edition.shtml
It will tell you just about everything.
With what your doing with your machine, I would want a fairly fast processor and and LOT of memory (ram). Historically a computer that is more than a couple of years old is really outdated. Things move so fast in the computer world now a days that toady's biggest and best, is tomorrow's bottom of the barrel.