1. Does it void the warranty to overclock a Sapphire GPU?
2. Is bitcoin mining a proper stability test for an overclock?
3. What are the upper and lower limits on GPU clock and memory clock for a 6950?
4. Is the greater risk when overclocking temperature or bricking the card?
5. What's the limit for a stable temperature for 24/7 operation?
1. No, so long as it is through software.
2. Furmark or the equivalent is better.
3. It's technically unlimited...you just have to try and see how high you can go without instability.
4. What? The only way you can brick the card is if you screwed up an unlocking process from 6950--->6970. You should be more concerned about potentially frying your VRMs if you try too hard of an overclock and insufficient cooling. If you are going for high clocks, either put it under water or get a high end air cooler with proper VRM cooling (Thermalright Spitfire and its VRM cooler)
5. GPUs are safe up to 105c, but it's never recommended that you let it go that high. I generally like to keep my temps under 85, if it goes higher I will lower the overclock and continue mining.