Well, I know that the 9500 Pro
is clock-locked unless you're running Omega drivers, but the card doesn't do it in a "normal" fashion. What happens is that you are allowed to reset the clock rate in software, but the card will ignore it and continue running at 276/270. Above certain settings, the drivers simply crash or artifact, as is the result when you try and underclock. From what I can tell, the official Catalyst drivers check the BIOS for the clock lock and then either simply refuses to acknowledge overclocking or outright crash the card.
The only two solutions are to reflash the BIOS with one that's had the lock removed (it's a two-byte sequence relatively early on in the BIOS -- just change them to match the same bytes from the 9700), or to run hacked drivers (that ignore the BIOS lock). I did the first, as I don't have to worry about always keeping compatible drivers, but it's a bit risky (if power dies during the flash, your card is dead, and that's all she wrote).
Here's an older FiringSquad article about the results of the hacked BIOS.
So far, I've only been able to drive up my speed to 336/306 before texture corruption and VPU Recover kick in, but I have only 3.6 ns RAM loaded on my card (it's stock ATI, while Sapphire and Gigabyte use 3.3 and 3.0 ns RAM, respectively, and can be clocked a lot higher). I can probably drive the core quite a bit higher, but that would be pretty useless without increased RAM bandwidth. Gaining the additional 5 FPS from OCing wasn't really worth the effort of reflashing and testing for me, though I've heard that people with the third party cards can get upwards of a 15% performance boost.
(EDIT: Caught a few typos in my initial post. Sorry.)
