We could overclock our 7900 GX2 cards to a maximum of 585 MHz core and 730 MHz on the memory which is an overclock of 17% / 21%. Very good.
Considering the design it may be possible that some waterblock vendors come out with GX2 water blocks soon, from an engineering perspective it seems not too hard to do.
With the overclock of 17%/21% the 3DMark 2005 score went from 12494 up to 12817, only a 2.5% increase.