Ummm hello...the Gt is not the overcloocked version of the GS. The GS is the UNDERCLOCKED version of the GT. Well technically not underclocked since the speeds are the same, just some pipes were locked. The card was designed to run better. Also I don't see how you can possibly have bad experiences with overclocking a card as you cannot damage a card by overclocking without a volt mod by using conductive ink. It can't damage it if it's running at cool enough temps, it just makes artifacts until you turn them down.
Overclocking most things are safe, I have my E6300 at 3.22ghz right now and video card are even safer. Trust me the 7900GS is defiantly one of the best value on the market. It doesn't matter though when the thing that will destroy both cards for the price of that 7800GTX is right here
If you have the money BUY IT as that's a sick card.