Hi people

I'm new to Bitcoin and video cards so please go easy on me if this question is silly.

I'm looking to buy an ATI 5870 and found that Newegg had 3 for  $200 after rebate. http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&DEPA=0&Order=BESTMATCH&Description=ATI+5870

I was wondering if there were any important differences between them, or between XFX and Gigabyte. More specifically my questions are:

Is there an advantage to the cards with two fans over the card with one? Does the extra fan make them run cooler?
Secondly, two of the cards have 850mhz core clocks and one has a core clock that runs at 870mhz. Will the 870mhz card mine quicker, or can it be overclocked more?

Cheers!