I have a related question to mobo choosing.  Wouldn't it be important to run these cards in x16 or x8 lanes?  If so, there's only one quad-card mobo with that support (ASUS Crosshair Extreme IV - $300).  Grabbing two mobos might be a hair cheaper with the ASRock 870 which is the cheapest board that supports x8/x8.  ASRock is kind of a new mobo printer but I think they're the shit owning one myself.  Next best value is ASUS @ $95 and Gigabytes crawling in @ $120+

So on one hand it'll be easier to stack them all up in their own box andbuying heat efficient boxes for 2 card systems will be a lot cheaper than a 4 card OC'er hotbox but then you can always build your own box like dude (LOVE THAT) and even build your own housing for it to counter the 'safety' issues.

Still, two x8/x8 rigs will only ring in $80-90 cheaper than the CrosshairIV (including an extra sempron) and also add power costs (not by much I'd imagine).

Anyway, basically i'm just wondering if the number of PCIE lanes available ot a given card will hinder performance.