It's not really fair to compare a lower card with a higher card because the lower card can overclock past it. I mean sure, the 7900GS is nice to overclock, but same can be said about 7900GT, I've also taken that baby quite far (I have the 512mb version, I took the core to 650 and stopped there because I couldn't overclock my memory that far due to inadequate cooling, so any more up would make little impact).
In the end you do pay for that little bit of difference between 7900GS and 7900GT - it just has a slightly stronger chip. Whether you think it's worth the price difference is another story. It's only $30'ish difference here, on an entire videocard that's probably just over 10%.