Number of GPUs is certainly the biggest factor! Nothing can touch the two-chips-on-one-card cards!
Number of shaders is how many simultaneous operations you can perform
Speed of core is how fast each one goes.
Clearly the last two work together to determine a cards potential hash rate.
What do you mean? My two-chips-on-two-cards (5830+6950) cost 1/2 of a 6990 and give more MH/s, less heat, less noise, less power use. ok they use 2 pcie slots, but for the difference in price I can get an entire new mobo+cpu+ram setup...
OP: in short, there's no such comparison for complete systems. You dont need a good cpu for mining, nor ram/video ram. You even downclock video ram to give less heat... It looks like the # of shader units determines the hash rates (and then the core speed of course).
Try to add up costs, and find whats best for you based on hardware prices/electricity cost. I don't care about electricity costs, so I run 3x5830s and a shader unlocked 6950. If you do, consider getting 5770s; they're the best in MH/joule. PSU is scaled to your PC needs. I have found that the best system for me is a small 2 slot mobo for 55 euros, sempron 140, 2gb ram, and 500W antec psu with 2 5830s.
If you wanna run a bigger system, check the prices of psus vs. watts they deliver. I really recommend using multiple psus (if you're running without a case), its no hassle to set up only takes 1 minute, and can be scaled better. Plus, most of the time, its cheaper at >900W.