memtest failure on x86_64

[Joe Smith]

I just built a new Athlon system and decided to test F9 on it.

Memtest86 from my old F8 rescue CD (i386 arch; memtest 1.70) shows all 
tests pass, and the system installed F9 and runs stably.

Except...

Yum update dies sporadically with database corruption, and memtest 
(v2.10) from the F9 beta install DVD image won't finish two tests 
without thousands of errors and it finally crashes. 100% repeatable.

Can you have such apparently serious memory problems, yet the system 
runs well?

The BIOS timing settings are still on factory defaults (auto setup).

The system is an ASUS P1-AH2, AMD Athlon(tm) 64 X2 Dual Core Processor 
4000+, 2Gb DDR2 800.

Any suggestions?

Especially, can anyone tell me for sure that the results from the F9 
x86_64 memtest are reliable? I seem to have conflicting indicators at 
this point.

<Joe