I'd budget 200 bucks for a base level PSU and water cooler. Sky is the limit from there.
I'd say this is a conservative estimate.
You're probably going to want at least an 80+ Gold PSU ~800w which will give you ~85% efficiency and will cost you ~$90-130.
Then, you're probably going to want a decent water cooling system with double radiator and two 120 fans and will cost you ~$110-130.
While you may get away with the bare minimums as far as cooling and power are concerned, I strongly advise that you acquire my minimum recommendations. They may cost you about 25% more than just the bare minimums, but they will save you money and headaches in the future. Do you really want to be stuck with just enough power output and then there's a firmware update that can increase hashing power by 15%, but you don't have the juice to squeeze it out of the board?
Not only this, but high heat on the dies 24/7 will only decrease their lifespan and max load on the PSU will only cause it to burn out faster and may cause damage to the boards, not to mention cost you more in electricity.
You can never have too much power and cooling available.
I agree bit_ninj4.
My numbers came from using a Gigampz kit for power (80) and a Nepton 280L. It might be best to use a PC PSU which has all the assorted connectors though. Some coolers require a sata power connector. And if you want to run more fans molex can come in handy. So maybe closer to 300 is better.
So 1150$ for 640GH/s doesn't look so good to ROI this investment. (almost 2$ gh/s) I don't understand why it would good to buy those in terms or ROI.
1$ for 1 gh/s and this would be awesome !
Otherwise great project !