Hi,
I have actually quite good success with two very cheap Powercolor 7990s. They have a design power of <400W (specs, not measured by myself) and they consistently deliver 1300kH/s each in an open-bench setup. If you go this route, make sure you have a 750W PSU (that's enough to run one 7990 card).
I picked these up at Newegg for ~$600 each. I feel I got a good deal.
Another thing you might want to consider is the power efficiency of your PSU. It's quite often overlooked as these cost more, but gold or platinum rated PSUs do really have a higher efficiencies
If you run more than one of these cards, you might need to use PCI risers to get the cards further apart. Otherwise one card will 'breath' the hot air from the other one. The hashing speed on these is on the one end limited by the heat, but also (if cooled really well) by the on-card voltage regulators. They limit the clock rate to about 1GHz GPU and ~1575 GHz on the memory. For short times it seems you can exceed these (I reached up to 1100MHz on the GPU and ~1750 MHz on the memory without damage).
Cheers,
Haku