You are thinking of upgrading a 3 month old 7800GT to a 7900GT?!? :roll:
What do you do for a living? How old are you?
I advise you to slow down, step back and ask yourself,,, Could this money be more wisely spent analyzing the market, technology and hardware before plopping down $400 and again and again?....
You just got a card, wish for another, and another and another... it goes on and on... And to top it off, you are inexperienced to the point you are scared of overclocking?!? You need to learn and understand your PC and it's limits.
Overclocking, when done by someone with intelligent research and advice (and in moderation) is not harmful to the hardware's life significantly.
You realize within a few months DX-10 cards (which are needed to take full advantage of MS Vista) are going to be showing up within a few months... Does this mean you will upgrade your 3 month old card now, and in 3 months again??? IMO, that is not wise, and you will always be searching for the unnatainable... And I suspect you do not have deep pockets, or else you would have gotten a better card (or pair) to begin with...
Take it slow and research what you have now and learn how to get the most out of it first. Then if you still want to upgrade, research some more on the direction you wish to go in and know what you are going to get and how you plan on using it first. Spend your money wisely.