You don't have to let it chug for 10 hours, it's stressing your ram subsystem hard, that's what it's for - it's not just a simple bitwalk. If it's going to throw an error, it'll probably happen in the first few iterations. (no need to run 100 iterations!)
re: after the computer's been running awhile, I don't mean leave memtest86 running, I mean when you're taking a break after a hard gaming or rendering session, reboot to that CD and start the test again... just the time it takes to eat food should be enough iterations to see if it's going to puke.
Also, nearly every RAM manufacturer offers lifetime warranty. So it's not like you have to get it done by tomorrow - if it's bad, chances are they'll replace it.