Those pre-built, self contained water systems are really not much different than a quality air system. Low flow pumps and a lack of reservoir really limit their cooling.
I actually just bought the Evo, I was leaning towards the H80i until the last second, when I realized, for $50 a couple degrees isn't going to make a big difference. Especially since the OC on a 4770K depends more on luck of a good CPU than cooling.
I still have a pair of 9800gtx+ cards churning away as well. I was thinking about replacing them with 660's, since the low power consumption on those means I wouldn't need to upgrade the PSU. I'll keep my nVidia cards going in at least one other box. Although, I have to say, after clearing up the error I initially had with the new core, I don't seem to be getting any more of the new WUs that use it.
I went from 8-10K PPD with the 9800gtx+ to 25-30k PPD on the old cores. I just switched to v17 and it went up to ~60k PPD. I'm not sure of the PPD difference between the 760's and 660's but the difference is 30w power wise. I only have a 500w PSU in my old rig otherwise I'd keep the 9800 going as well. Once I finish buying the parts for the new one I'll have both folding as I eventually plan on getting a 2nd 760 as they're only $250 and I'm sure will drop a bit later.