The H2O 620 is a simi water cooler (or a "all on 1" water cooler) and it should be plenty to keep that CPU cold even with some overclocking.
so don't worry about it.
Should you SLI a 670. well, if you got the money and need for it then yes.
personally i am not much for SLI due to more heat and noise. i rather have a single better performing card.
i am going to buy a GTX 770 when it comes out later this month.
you didn't think about waiting a little bit and get the 770 instead?