Rember uses memtest at its core. However, Rember isn't able to test any memory that is being eaten by the OS, so, if you want to really test your RAM, you should:

1) install Rember

2) Boot into single-user mode:

hold command-S on boot, then:

/sbin/fsck -fy

/sbin/mount -uw /

3) Once you have the hard drive mounted, run the memtest executable buried in Rember. If you installed Rember to /Applcation/Utilities the path would be:

/Applications/Utilities/Rember.app/Contents/Resources/memtest

For memtest switches look here:

memtest 4.21 user guide