Understanding the Difference in The GTX 285 Choices

Could someone please provide some feedback on why the GTX 285 cards vary so much in price? In particular for the ones that have the same level of onboard RAM. I know prices for manufactures differ, as well as if its factory OC'd or not and how much onboard RAM they come with. The higher end cards in that lineup list Stream Processors: 480 (240 x 2) Processor Cores, compared to the lower priced 285 cards with just 240 Processor Cores. Would the 240x2 mean that card is designed for dual monitors? I'm running a single monitor and 1600x1200 res on a CRT. Could go higher if I wanted. Not sure if the 240x2 would be worth the cost in my case.

I also notice that the Core, Shader and Memory Clocks MHz's vary from model to model. Is mainly just from how the manufacturer chose to set it? Could you just buy the lowest model 285 card and OC it yourself to achieve the performace of the higher priced 285 cards? (not considering the difference in the RAM on the card). Or are there other things to consider?

Right now I'm looking at getting one card with intentions of going SLI in a few months once the prices comes down more. Do you prefer XFX over the BFG and EVGA line?