Update 2: Not so good.

One of the modules was faulty but only really started playing up after I updated to 10.8.2 so, for ages, I thought it was a software problem and had been trying to reinstall OS X, which just [censored] everything up completely!

My question is... is it possible to create a bootable USB drive for memory testing? www.memtest86.com seems to offer a way of creating such a USB drive but only from Linux. Is there an OS X solution?

Never had an issue with Crucial even when returning RAM because I thought it was causing strange behaviour when waking from sleep (the machine was spontaneously rebooting). They replaced it with no questions asked and the new RAM worked without problems (proving my case).

It is worth noting that they do always religiously follow Apple's specs, which are often not what they appear to be so it is worth checking online what your Mac is capable of in terms of maximum RAM and specs not what Apple say it is. To prove that point I'm using an original unibody Macbook here. Apple and Crucial will tell you that it maxes out at 4Gb (2x2Gb), but it uses 8Gb (2x4Gb) just fine. I had the same deal with a Powerbook that could take double the RAM Apple and Crucial said it could.