With the memclock set to 300 the program reports back a speed of 1250. Lowest I can go where it reports the value as what I specified seems to be 800mhz for the memory. ...
This is a known issue with some (maybe most) 6xxx cards. I think the problem is in the Windows driver so if your on another OS then this might not apply.
The 6000 series cards are extremely unstable at some
reduced memory speeds. So in order to prevent people from running their mem in one of these ranges, they just won't accept any value less then 100 or 125 MHz lower then the core speed. My 6850 wouldn't accept anything lower then Core-100 but I heard many people saying the limit was Core-125.
topheroly clued me in to a the workaround a while back. I can't figure out how to quote a different thread so please just follow the link: https://bitcointalk.org/index.php?topic=33276.0
So after playing around and finding some memory speeds that didn't result in complete graphics corruption within seconds I set myself some preset speeds. So I could launch MSI Afterburner, pick my first preset, close it, relaunch it, pick my second preset, close it, relaunch it, pick my third preset then launch my miner. After confirming everything was running fine I could close Afterburner but would usually leave it running for the monitoring panel.
In the end I got tired of playing with afterburner. Sold my 6850 and bought a 5850. Even undervolted and sporting a more modest overclock, it takes more power to run then my 6850 did but the 50% increase in shaders means I get more MH/J then before the switch.