Updating OS leading to MacBook not turning on.

I got my MacBook Pro December 2010 (so it is no longer under warenty) and the OS disk contains OS X 10.6.3. I had issues starting my computer a few months ago and the only way I was able to get my MacBook starting again was to reinstall the OS. This happened again about a month later after trying to install updates, and agani I had to reinstall the 10.6.3 OS from the disk. The current version of iTunes requires 10.6.8 or Mountain Lion, and I wanted to add new music to my iPod so I tried to download Mountain Lion, only to find out I couldn't get a disk and instead had to get it from the App Store, which 10.6.3 does not have. I tried to download 10.6.8 only to have it tell me I needed 10.6.5 to download that, so I downloaded that and then installed the following updates: 10.6.5, 10.6.6, 10.6.7, and finally 10.6.8. After installion 10.6.5 my Mac was unable to start up past the white/grey Apple screen, so the rest of the updates were installed in Safe Boot mode. Did this happen because I skiped 10.6.4? If I buy Mountain Lion right now will I still have this issue and have to reinstall my 10.6.3 disk, wasting $20? Am I just stuck with 10.6.3? (Before reinstalling the OS I try Safe Boot and restarting, Disk Utility, Fsck, resetting the NVRAM, and then as a last resort I reinstall 10.6.3). I've got a Time Machine backup on an external hard drive from right before I started doing the individual updates so I'm not concerened with memory loss.