A good idea.

How to Test RAM Under Mac OS X | Command-Tab

Memtest Usage To run memtest on a new memory module, first shut down your computer and install the new chip. (Some helpful guides for doing this can be found at iFixit, if you’re unsure of the exact steps.) Ensure the chip is firmly in place, close up your machine (or don’t, if you’re a pessimist), and power it on while holding down the Command and S keys to force Mac OS X to boot into Single User Mode. Once you see a black screen with white text, you can release the key combination. After all the system logging is done scrolling past, type memtest all 2 to test ‘all’ memory ‘2′ times. Two passes should be enough to detect any blatant problems, but I wouldn’t hesitate to let it run for hours on end if I suspected an intermittent memory problem (memtest all). When complete, you should be greeted with “All tests passed” if your new RAM is in good condition. If your system locks up or freezes indefinitely during the test, you may have a bad memory module on your hands.