For anyone who has been looking for a solid, easy-to-use command line tool that properly underclocks a 6990's memory under Linux, please take a look at this thread:

http://forum.bitcoin.org/index.php?topic=25750.msg320458

This atitweak tool is great and is the only one I've found that will allow me to lower my memory clock from a command line on a 6990 without a hassle.  It's a very solid replacement for aticonfig and/or AMDOvDrvCtrl for things like over/underclocking, fan control, GPU status, etc.

My 6990s now run a lot cooler since I've been able to underclock the memory.

I can't take any credit at all for anything except stumbling across the right thread.  If the tool helps you, flip the guy some coins.