To test your CPU/MB enter the number of threads supported into the [threads] box, and choose an amount of RAM to test that is comfortably less than the total available. If you check []Low priority threads you can even use your computer while testing. This is a good test for overclocking/stability since it's more important to work each core to the max than to touch each byte of RAM. 

To test all your RAM, first maximize available RAM: quit everything (or better yet reboot). Each thread can test about 3500MB, max. If that value multiplied times the number of CPU cores is less than the available RAM, then use more threads. Note: you can leave [MB per thread] empty and MemTest will automatically grab all the memory it can. Be sure [] Low Priority threads is not checked, or Windows will proactively reclaim memory from MemTest, vastly slowing the testing process down. NOTE: you may find that after the test has run a while the amount of available RAM will increase significantly because Windows has swapped out idle programs to disk. Restart the test to grab their RAM, but leave 20MB or so for Windows, or it may start swapping, slowing the test down.