Check out the following screenshot from an 6600 card.
The core slowdown threshold is set to 145C where a typical CPU would get destroyed above 100C.
That means that vga cards can handle much more heat so don't worry if you see similar temperatures.
When the 6600 reaches 145C it will automatically decrease the clock speed of the core so that it drops to a safe level.
I believe all modern cards that have a built-in heat censor like yours do the same thing.
Also just like fernandes said, if you're not seeing any kind of artifacts in the game and your computer doesn't hang when playing games for more than 5-15 minutes you shouldn't have any problem and your overclocking should be considered successful :)