you'll need a 1000-1200watts PSU for that kind of machine.
Also, according to the latest tom's charts, 2x 4870x2 doesn't really do anything special up until 1920x1200. If you game on a 30'' display, then it'd be a decent idea but anything below = you'll have the noise of a vacuum, the heat of a furnace and the electricity bill of a small town.
As for air cooling vs watercooling, you also have to remember that the x2 requires either 2x mcw60 blocks with special DIY mounting equipment or a 150$ full plate block. That's on top of the rest of the loop which will cost around 250-350$. On the upside, it'll be less noisy.
For the moment, overclocking the i7 chips seems to be limited compared to the last round of chips, so heat shouldn't be as much as a problem. I'd stay with air myself, as its cheaper and easier to set up, as well as having less maintenance problems. As antiacid noted, water should be less noisy, but that's not a guarantee, especially if you use quiet fans for the air cooling.
Also, as previously noted, with those big video cards, you would probably need a 1000-1200wt PSU, and maybe larger. If you actually make such a machine, consider having a dedicated electrical outlet for the computer only, perhaps using a heavier amp rating on the line and circuit breaker as well. That's a lot of power to pull through the standard production lines.