Overclock is all about temperature and stability. If the card runs stable in benchmarking and doesn't overheat, or run just about at the edge, then you are fine. So continue to push the voltage and mhz up. If voltage is locked you may be at the limit of what it allows you to handle.
One thing to keep in mind is every chip is a tiny bit different, so for example if one guys gets 1000 mhz out of his board you may only be able to get 990 from yours Thats why pushing it and testing is so important.