230 at stock vs 175 at stock. 79xx has the automatic memory voltage control, but has a limit of 150mhz lower than core. However, at, say, 1125mhz like mine is, moving memory from the stock 1375 down to 975 cuts down on GPU temps about 5c. On my 5850, which didn't have automatic memory voltage control, I had to drop the 1000mhz memory down to below 300 to get the same 5c drop.
Don't forget that you can run OC mode 2 on Afterburner and set the RAM to whatever you want.
My 7970 is currently at 1185 core/710 mem, hashing at 709 Mh/s stable. I've found that if I drop the RAM clock below 710, it gets a bit unstable.