Is it that your just putting the slider to 1,000 clicking apply and that's how you're determining that you can OC to 1,000mhz? If that's how, that's not the way to overclock a card, nor is it the way to overclock anything for that matter.
Raise the Core clock 9GPU clocks not memory) by 20mhz steps and test with something such as 3dMark, Heaven, ect. that will put 100% load on the GPU. If it is stable through 15 - 20 minutes of testing, raise it more. When you start to see artifacts then drop it back 20 mhz and do longer test. Once you have the highest stable core clock, work on the memory.
You would only really be able to "burn out" the video card by too high of voltage. Before messing with the voltage, I would recommend reading up on it a little more. Overclocking isn't a race [unless you're doing championship level] - take your time with it, once you get stable settings, you will be able to leave them there and be happy with it!