|By process of elimination you've verified the ram slots and ram sticks are all good. They're just not working together. When googling that error it seemed to occur more often than it should with HP's so maybe it is a bios or motherboard problem associated with their architecture.|
I checked the support page for that model:
for a bios update but they didn't show any. I did notice you could order the recovery disks for it. If that's not what you're using to do the reinstall you might consider ordering a set from them. I've done it once or twice. As I recall the most it ever cost was around $15. However I don't think that will fix the error since it's showing with memtest too.
One solution to the error was to make sure the ram slots are clean and the sticks firmly inserted. But that doesn't seem likely in your case with all the testing you've done.
I think you should verify the ram you're using exactly matches the specs provided by riider and Fingers as it may be possible that although the sticks are both good they're not compatible with each other, at least with HP's. Or you could try replacing one of the 2 gig sticks with a 1 gig stick and see how it runs with 3 gig.
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