I suggest spending 50 more, and buying 2 mobos, psus, cpus, hdds, and 4 5830s or 3 5850s and get 50% more hash rate at only 50% more power consumption.
That's semantics, you can't get those cards new & boxed anywhere at reasonable prices anymore, or at all since they are 99% out of stock. The bulk of my rigs contain 5830's and 5850's and I bought those ~9 months ago..
Even then it was a struggle, and I had to settle for a few used ones.
Even eBay is mostly filled with used ones or massively price gouged pieces. Newegg ran out ages ago.
The only realistic purchase option at this time is a 6870 or a 6950/6970 if you go for ordinary cards instead of dual gpu's.
But I do agree that if you run your cluster as a home op, even a single 6990 will make your room uninhabitable.
I'd never do it at home, but I do run 4 at separate location.