I've used memtest86 in the past.  It seems pretty decent, though
I don't have anything to compare it to. :-)

-Andrew

http://reality.sgi.com/cbrady_denver/memtest86/

From: "Jenkins, Graham K [IBM GSA]" <Graham.K.Jenkins@team.telstra.com>:
> I just took a look and found that video-RAM shadow was enabled.
> Would that have caused it?  And does anyone have a good memory-test
> program?
>
> G.