Hi I have a PowerBook G4 with a FireFace800. To confirm what someone else said, when I play my Ivory Piano, I have a hard time reducing the Latency. I also get craks and even 2 second glitches here and there. Of course, according to barefeats.com, the PowerBook I have (the last PB G4 model) has a lower bus through the pcmcia slot than the previous models! For the FireFace400, it seems that how low you can reduce the "Limit Bandwith" is important. if RME will allow you to go lower with the FF400 than the FF800, than that would reassure me in buying the FF400. The more channels you've got on FireWire, the more bandwith it takes and the more likely you may have problems. The FF800 setting has these 4 Limit Bandwith, the lowest setting is 8 analog channels. 4 times more than on the your MacBook or PowerBook stereo output. That's why according to RME, the Apple Stereo output will give you lower latency and less cracks! If you have more news on the FF400, please let me know as i'm interested in that one too, instead of my FF800.