Underclock GPU Memory below "Recommended" Limits on Linux
aticonfig respects the settings set in your video card's firmware by your card's manufacturer (frequently, although incorrectly, called a BIOS). However, AMDOverdriveCtrl can clock memory down much farther, depending on the card.

Radeon 5xxx, 67xx, and 68xx series cards should be clocked down to 1/3rd of the core speed (ie, 750mhz should have a memory speed of 250), 69xx cannot clock down far enough to be worth it, and GCN (79xx, 78xx, 77xx) seem to have a limit of 150 mhz less than core (however, although it is not worth underclocking on 69xx, it is worth underclocking on GCN due to automatic memory voltage control done by the hardware/driver; on my 7950 @ 1125mhz, going from 1375 down to 975 dropped my GPU temps 3-4 degrees celsius).

5xxx and 6xxx series cards also have a problem where they will instantly lock up if memory speed is set between around 400mhz to around 600mhz (depending on core speed), so your mileage may vary as always.