If you've run memtest and it's been spitting out errors, stop posting on web forums and get your memory replaced.
Generally, on the first error that shows up, it's fine to stop the program.
If you need to isolate which stick is causing the problem, take the others out and test them one at a time. If it's a memory channel kit (3 DIMMs in one package for example) then go ahead and just have the maker replace all three because then they can usually guarantee that the new set will be flawless.
Also, your signature doesn't say what kind of power supply you're using. Low quality power supplies can introduce problems where there are none. In this case, a weak power supply can cause even good high quality RAM to throw errors in MemTest.