RAM issues

What a day. When I embarked on the reinstall fest, I never thought the following would happen:

Reinstall all working software and OS updates -- OK
Reinstall World of Warcraft -- failed on disc 1
Clean all 5 discs failed on disc 3, different issue
Tried WoW four more times -- failed on different discs every time, never getting past 3rd disc
Booted to CD, ran disk utility -- extant allocation errors (same thing that I got last week)
Erase and install, update os x, run disk utility -- same issue
Removed windows partition with Boot Camp Assistant, reformat entire drive, run Disk Utility-- no issues
Erase and install -- Got to desktop
Installed 10.4.9 update and rebooted -- now cannot log in, password is rejected (nidb looks hosed)
Archive and Install NOT preserving, new user account -- now can log in
Disk Utility -- extant allocation errors, AGAIN
Apple Hardware Test: 4MEM/1/40000000: (error message)
Verdict: flaky RAM or mis-seated RAM, do not have a screwdriver small enough to reseat the memory
Set genius bar appointment for 1130 Monday at Arden Fair, sent system profile to Apple.
Replaced torn out hair with a bad toupee

I can understand how bad RAM can corrupt a hard drive; I see it all the time. At least I have enough functionality to run os x and do basic stuff, get on the internet, etc. Just cannot install or play WoW. At least in OS X . . . .
OK, here's what *I* do not understand: How does a system pass Apple service diagnostics yesterday in the store but fail Apple Hardware Test at home?