Ok in a nutshell
they are both the same card just a different heatsink on them
the dual Heatpipe version is the one selling with the XPS M1710 it allows better heat dissapation and therefor should allow for better overclocking. It does however require the person installing it to Nip 2 small metal tabs (check some of the posts in this forum) In the end the dual heatpipes will keep it cooler and allow a longer life to the card and an overclock that is not possible with the single pipe version.
Single pipe is just that its only got one cooling pipe and its the one that is shipping with the e1705.
Now taking into consideration that the only difference between the 7900gs and the 7900gtx is a laser locked set of quad pipes my supposition is that the 7900gs wilth the dual pipes should be able to clock just about as fast as the 7900gtx because it is the same exact chip.
I hope that helps