Hmm- well, I know next to nothing about Macs, but let me make a few general observations:
Crackling and popping in a digital recording generally comes from one of two things: clipping or timing issues. If you're sure it's just the drums (not, say, vocals or guitar), then I would suspect some kind of clipping because drums are loud and have very strong attack transients. I would not rely on a meter display to check this, I would look closely at the wave forms.
If the noise is present on everything you record (and you've checked your recorded levels carefully), then it is probably a timing issue. This could be a buffer issue, or it could be a resource issue. Since you've tried adjusting your buffer size, I'd lok at resources or timing with the firewire port.
But don't forget my first point- if it's ONLY with the drums and no other instrument, then the problem is most likely with the way you're recording the drums, not your computer setup.