He's talking about taking the heatsink off and replacing it with a watercooling shroud, which I highly, HIGHLY doubt is covered in ANYONE's warranty... except perhaps EVGA, but that's just a guess.
OP, the only difference would be that the superclocked cards might be higher binned - I know it's true for the 7970 GHz editions. That means you'll hit a slightly higher overclock, but I don't know if it's $50 more each card worth of higher overclock.
As for buying two 680s, don't do it. A 680 is only 5% faster at STOCK than a 670... that goes down to 3% after overclocking both on air, and it'll be even less on water. That's nowhere NEAR worth $100 to $150 more per card.
By the way, what is this going to be used for? If it's triple monitor gaming, go with the 7970's; if you buy 670s, you'll need the 4GB versions. If it's not triple monitor gaming, I'm sorry, but you're being a bloody fool; a single overclocked, air cooled 670 or 7950 will max out any game out there.