When it comes to overclocking, much to most manufacturers horror there's only one option; MSI Afterburner. The fact that MSI let you use the program on both NVIDIA and ATI cards from all companies means that anyone overclocking is probably using it.
When it came to voltage we thought since the cooler did such a good job on the card with it running cooler than previous stock cooled HD 5970s, we would just crank it up to 1.3v. Once you hit Apply it drops back to 1.299v, so that's what we got set on.
Out of the box the HD 5970 TOXIC 4GB from Sapphire is giving us a core clock of 900MHz. With the above voltage we had no issues running at 1000MHz. Unfortunately we couldn't go any higher; when we did it just bounced back to 1000MHz in Afterburner.
The memory clock was also able to get in increase of 59MHz to 1259MHz; any higher and we started to get freezes in our benchmarks. 1259MHz results in a clock of 5036MHz QDR thanks to the GDDR5 memory. You have to also remember we're talking about 4GB of memory here as well, instead of the standard 2GB we see on other HD 5970s.