Thanks, atm 600hz!
Ok, so, it seems I actually had to do this for my 3X6970 rig instead of my 5870. My 5870 had no issues dropping to 170 mem using CG miner. However, my 6970s aren't reference cards and initially it wouldn't let me drop my mem clocks more than -125 below my core clocks. Using the following method, I was able to get my mem clocks down to 685. Maybe this will let you drop your mem clocks below that for your 5870.
Make sure you have the latest MSI Afterburner Beta, then search for the msiafterburner.cfg file.
I right clicked the .cfg file, went to properties --> open with notepad --> adjusted "unofficialoverclockingmode = 2" (was previously zero) --> save (I have 'hide file extensions' unchecked) and then opened MSI Afterburner.
So, essentially you're changing "unofficialoverclockingmode = 0" to "unofficialoverclockingmode = 2" and then saving it. Then, reopen MSI Afterburner and see if you can drop your mem clocks any lower.
If you can't, I'd still recommend using CGminer to make sure. With GUIminer, my 5870 was running at about 400 m/hash even with massive overclocking. CGminer gives me a better hashrate than GUIminer all else being equal. There's no way I'd be running at 456 m/hash with GUIminer.
Edit: Actually, I'd probably try CG miner first before the MSI Afterburner trick. You can change your clocks, fan speeds, and voltages all through CGminer and it gives a better hashrate than GUIminer for the 5870 anyway.