edit: Okay, the memtest home page says:
There have been numerous reports of errors with only tests 5 and 8 on Athlon systems. Often the memory works in a different system or the vendor insists that it is good. In these cases the memory is not necessarily bad but is not able to operate reliably at Athlon speeds. Sometimes more conservative memory timings on the motherboard will correct these errors. In other cases the only option is to replace the memory with better quality, higher speed memory. Don't buy cheap memory and expect it to work with an Athlon! On occasion test 5/8 errors will occur even with name brand memory and a quality motherboard. These errors are legitimate and should be corrected.
My error is happening on test 5. I know that the trouble causing RAM is a PC2700 DDR stick, which is listed to work with my system. It says to set more conservative memory timing - how would I go about this?
My bios is AwardBIOS, if that helps.