Watercooled 680 vs WC 7970

yes, the 7970 gains more performance when it is overclcked compared to a nividia 680. they are about the same when they are both overclocked to the same level. the big thing about the 7970 will use much more power when you overclock it compared to a 680 OC.

yes, it will matter as you change the fans for a water block as some models come with a custom PCB. Custom PCB generally have more power delivery coponents and they allow for higher clock speeds. if the PCB is refernece then it would not matter

Im pretty sure tht EVGA offers great warrantis on their cards. i do not know that much with the AMD cards

i also recomend the EVGA hydro copper cards as they come with a preinstalled water block and they look nicer

I don't care that much about looks as I am using an old modded radio from the 70's as a case so most of the time I won't see anything   I know they overlock roughly to the same levels on air, but shouldn't the 7970 be able to overlock to a higher "level" on water?
Also, does the adaptive vsync from nvidia provide any real benefits?

Adaptive VSync review:
http://www.hardocp.com/article/2012/04/16/nvidia_adapti...

In my research, I'm generally seeing the 680 hitting higher overclocks than the 7970. At Guru3d, they had the Asus GTX 680 DirectCu up to 1227 mhz with a turbo boost clock of 1367 mhz.
http://www.guru3d.com/article/asus-geforce-gtx-680-asus...

While the MSI 7970 Lightning was only clocked up to 1190 mhz with additional voltage.
http://www.guru3d.com/article/msi-radeon-hd-7970-lightn...

In fact, KFA2 is coming out with a factory overclocked GTX 680 LTD OC at 1202 mhz and a 1267 mhz turbo boost out of the box. That's a rare volt-modded extreme overclock on a 7970.
http://www.guru3d.com/news/new-kfa2-geforce-gtx-680-gra...

Here is a roundup of several custom 7970's where only one was able to hit 1200 mhz with maximum voltage (check out the rest of the review, it's pretty cool):
http://www.behardware.com/articles/853-18/roundup-the-r...

Here is an OC vs. OC comparison to show how performance scaling varies with the game:
http://www.xbitlabs.com/articles/graphics/display/nvidi...

Last question, would there be any performance difference between getting an hydro cooper card or installing a full block on another card? ( they both have reference PCB)

They should both be very similar in performance, depending how much better the full block system is.