mem throughput versus DRAM clock speed???

Using memtest86+ to test some generic ram. System is MSI K7N2 Delta, AMD3000+, 2 x 512mb dual channel DDR400. With everything in the bios set at default settings (timings are 2.5-3-3-7 using SPD), I get a throughput reading from memtest of 1135mb/s and test5 errors
I change only the cas setting to 3 (mem timings are now 3-3-3-7) and on reboot the DRAM clock drops to DDR333 (167mhz) and the mem throughput leaps up to 1195mb/s with the system stable as a rock.
What is a better rating, the throughput reading or the DRAM clock speed?
Also I can't figure how backing off the cas value causes the throughput to increase. Can anyone explain this to me?

Thanks