What kind of PSU are you figuring in with that though? A good PSU is going to set you back and for those 4 cards you're going to need 1500watt or more probably.
My two 5970's draw 770Watts when overclocked to 900Mhz, or around 650 Watts when at factory clocks. So 1500Watts if left at factory clocks is going to be plenty, but two 1000Watt PSU's would give him the advantage of overclocking: Each 50Mhz yields 50Mhash/s boost on each card (25Mhash/s per core). So overclocking both my cards by 150Mhz to 900Mhz gained me 300Mhash/s more. However, your going to need some serious cooling to get these clocks (Clocking the memory to 500Mhz helps).
There are simple and cheap adapters you can get that allow you to easily use more than one PSU on your system: See here http://www.kustompcs.co.uk/acatalog/info_1919.html
I'm not sure about the best way to go about it though. The power isn't shared evenly by doing that right? Whichever PSU goes to the mobo is going to use 300watts for the pci-e (75 each right?) and the board / cpu. Then you've gotta split the cards up between the two PSU's. I'm guessing you could go with 1 card / mobo / cpu / other on PSU1 and 3 cards on PSU2. I'm just debating this or 1 big PSU, not sure which is the best way to go, 1PSU gives you better air flow etc.