I found that some cards will just not let you do this without flashing a new BIOS. I have build 3 rigs (2x5870, 2x6870, 2x6950), and while I can set clocks on the (XFX) 5870s, I could not on the (Gigabyte) 6870s and the (XFX) 6950s. For editing the BIOS and reflashing your cards search for RBE (Radeon BIOS Editor).