Your memory timings are really loose and that is almost certainly affecting your memory benchmark scores. Are you still running at the default memory voltage? Have you tried bumping it up and seeing if that lets you tighten the timings a bit?
Also, a lot of 875P boards had memory stability problems at PC3200 at the launch. Abit has updated their BIOS a couple of times, and the latest (v1.2, I think) is reportedly a lot more stable at high memory speeds.
BHD