Hehe, a TNT2 is basically the same thing as TNT1, except TNT2 has a faster Core and Memory.
I recently overclocked my TNT, Its running at 121/140 right now, completely stable.
I suggest just overclocking as high as it can go, before you start seeing artifacts.
Use 5 mhz increments. Start with your memory first, Go up and up, until its unstable, or you see artifacts. Once you do, bring it back down 5 mhz, if its still unstable, bring it down another 5. Then do the same with your core, and there you go, a perfectly safe Overclocked TNT2.
BTW, who ever gave you that gibberish about 10% Higher isn't safe/recommended is total BS. Overclocking isn't safe at all, and it does have its effects, if done unproperly w/o decent cooling, you can damage your Videocard/CPU. Careful Overclocking is the best way to do it. Don't try a 3000mhz core on your TNT 2, because its not going to happen, and you're gonna fry the thing.
In conclusion, overclocking is as safe as YOU make it. Take it carefully, and you won't damage anything.