It's entirely up to whatever specs you can afford/fit in your computer case....
I've got two 6950s in crossfire. They're overclocked to over 6970 speeds and shader unlocked, so for the purpose of discussion, they put out as much heat as at least 6970s. I've also got a 2600k overclocked at 4.3ghz.
I use 2 XSPC Razor 6970 blocks which keep my cards at under 50c at load (in everything but furmark, but furmark is a bad measure for temperature anyway since it's entirely unrealistic), and a Swiftech GTX for CPU. My radiator is a HW Labs GTX360, and my pump is an MCP355 with XSPC res/top. I've got 3x2200rpm Yate-Loon fans on a PWM controlled controller.
With that hardware, I'm at about a 5c air-water delta, which is very good. It would likely provide extra overhead for 2 more GPUs before I would have to consider adding a second radiator. I rarely have to crank up my fans, and based on my own fan profile, they never go over 70% speed. I went watercooling because of the noise of the GPU fans, and because it's fun to tinker with. Lower temps have been great, and I've had a great experience thus far as my loop has been easy to set up and run.
Check out my build log in my sig for more info on my setup, and feel free to ask any questions since I've got a similar setup to what you're setting up, except you'll have one more 6970.