overclocking can create a lot of extra heat, as long as u dont say oc memory thats at 800mhz to say 1500 lol then that shud be the only cause of damage i think? correct me peeps if im wrong. Also the component tends to have a reduced life expectency, but it will still serve way beyond its usefulness. Sooo as long as u have good cooling then it shud be ok, just keep an eye on the temps when its idle and under load, any sign of it getting too hot, bring the oc down.
a lot of people will prob say to wait for DX10 cards to come out b4 u upgrade, but personally with the wait for them, and the games to support it i think ur better off getting a card now. i have the nvidia gforce 7950 gx2 and it runs anything u throw at it. but the 7900 is also a good card, and i dont think u shud get SLI personally for the 6800GS, as the card is old now, better performance probably from a single 7900 or at least u can then get sli for that when u can afford it.