advice on how to overclock 6870 on ubuntu 11.04 32-bit linux

[beeph]

I've seen alot of threads regarding 6990 and other cards. 

I would like advice on what utility to use, what clock/core settings to try.  Where to get the software too!

As of now I'm running around 250 MHash with default settings.  What kind of speeds can i get using just the amdoverdrive tool, without flashing BIOS

[beeph]

I got AMDoverdrivectrl installed but seems very limited in what i can do with it.  cant underclock memory more than 100Hz under the CPU, cant take voltage very high either