Sapphire 6950 1gb overclock

Yes I think so.

Yes the temps and voltage are within safe limits, I wouldn't recommend exceeding 90C for the Core or VRMs and I wouldn't increase the voltage past 1.25V for 24/7 use.

That is very reassuring. I have tried higher overclocks also something like 950/1325 at voltage of 1.27 mainly for bf3. I could not go at higher clock speeds for memory. It just crashes. Why is there a risk of running the card at higher volts even if the temperatures are within the acceptable limits ?

Well the main risk with overclocking anything is that electro migration will cause damage to the product.

Here is some very in depth information about electro migration if your interested: http://en.wikipedia.org/wiki/Electromigration

Basically too much heat will cause damage via electro migration but so will excessive voltage. Lower temps will allow you to increase the voltage more so, that's why people that use LN2 can get away with using extreme voltages.