arctic silver paste is just that: paste. it's not designed to hold the heatsink on, just to provide a thermally conductive medium to interface the heatsink, and the heat source.
the thermal pads are designed to hold the heatsink on too.
if you use just the Arctic silver, you run the risk of the heatsink falling off (and another rma scam

).
i understand you must have a Twinplex and separate heatsinks for the memory? or do you have the waterblock specifically for the 6800gt? if the latter, i'd personally use Arctic silver on the memory chips too, as the waterblock is held on with the plastic screws, hence you don't have to worry about it falling off!