1. Is my PSU overkill for my system?
2. If the answer to question one is No, will it also power a 2nd 6870 and a CPU water cooler?
3. If the answer to question 2 is Yes, would it be recommended to upgrade to the 850 or higher?
Keep in mind I would like to have this computer somewhat future proofed and I am looking into the possibility of overclocking and using 2 video cards in crossfire and a cpu water cooler. Currently the computer will only be used for Starcraft 2 and general web browsing like forum posting and such. I will likely be playing a few new games here and there when they come out as well.
The psu is definatly not overkill for you , I don't think you will have any problem with another video card . However if you add water cooling then you have to take into account the pump and however many fans you will have on the radiator you select. When it comes to the electrical components in your computer it surpriseing how fast the total adds up and the thing that slips through the cracks a lot are the fans , you have to add up all the fans and lights and everything that is pluged into the Pc.
Each 6870 under max load draws ~138W or ~12 amps. on the 12v rail. The Corsair PSU claims to have 62 amps. available on the 12v rail. So if this is the case you'd have plenty of power for Crossfire, overclocking, etc. if you figure 18 amps. for the CPU and other 12v power consumers.
Yes you could add a H80 cooler but I'd recommend a quality air cooled HSF over the Corsair H2O coolers which are inefficient and noisy. See the second link for a comparison of quality air and Corsair H2O water cooled heatsinks.
1- do a google search on the H60 or H100 and you will see that they consistently outperform air coolers. H80 is shorter and fatter, H100 is longer and thinner. H100 just performs a little better than the H80.
2- you almost always replace stock fans on any cooler (air included) if you are really looking for max efficiency and low noise.
wait for a rebate offer like a few weeks ago and you could get an H60 for ~$50 or so or an H100 for about ~$100. The H100 was $91 on NewEgg just last week. Not sure about the H80 but they probably have rebates also.