nothing is stopping you from setting the clocks that high. The GPU is capable of running at that speed, but the AW cooling solution may not be able to keep up with the increased heat output. A cooling pad would prolly help if you wanted to try going that high.
dunno...start with either the mem or engine clock, only one of them, and increase by 5MHz and then test for artifacts. Keep repeating until you see artifacts and then back of by 5Mhz, then switch to the other clock. I would imagine that you could get at least 25MHz out of each one, but don't just crank up the clocks...DO IT RIGHT.
They are "visual glitches", for lack of a better desciption. Usually caused by minor overheating or excessive clock speeds, the vid card will display large geometric "blurs" during moving scenes such as games. Use a game or benchmark program to check for these while adjusting the clocks. They are tough to describe with words, but when you see one, you will know exactly what I'm talking about.