As long as your not upping the voltage you will not shorten the life span. Ideally under load you should be under 80 celsius. So once you hit a little under 80ish probally stop. Start by increasing by 25Mhz and testing if stable and repeat until satisfied. Also use sapphire Trixx. Its Awesome and a easy to use! you can also control the fan in it. I have the sapphire 6850 and it can OC like a boss. Good luck!