Eh I'm pretty sure clock speeds aren't going to brick your card. Voltage and heat will do it.

That being said, I put together my 3x 6950 setup with linuxcoin and ran into the same problem. I couldn't get the memory to clock under core - 100. I disabled CCC Overdrive and it didn't help. People said Catalyst 11.6 fixed that (maybe it did with 5000 series cards) but it didn't do me any good.

The only guys who seemed to be getting anywhere were the ones who flashed the bios to another manufacturers (I think Asus) or modifying the bios to change the min/max speeds.

http://forum.bitcoin.org/index.php?topic=19776.msg290674#msg290674