Given all the costs you need for insane case, solid cooling, then the risk you're taking of slamming all of that heat/power into such a tight space, I've found it is smarter to just build out 2x seperate machines.  Decent off the shelf cases that are sub $60 can handle 2x 5970's rather well.  The asus EVO motherboard is fairly cheap but works great.  I spend about $100 more than you would spend trying to force it all into a single setup.

Also side bonus, if for some reason the box eats it, only 1/2 your mining power is down.