don't use ntune, it's one of the worst programs you can use.
Is it possible to overclock and kill your video card
Seriously think about that question dude. If you've read into overclocking at all, then you should know it's common sense that theres always the risk of permanently damaging hardware.
Bump up in small increments, watch your temps, run 3dmark03 and 05 and watch for artifacts. Do these simple things and you'll be aright. Use coolbits or (my personal fav) Rivatuner for OC'ing your vid card.