I think that it all starts with the stability of the system. The fact that Unix has protected memory, makes it much easier to load a 2-3GB file and trust that it's going to work, without the computer crashing on you. At my work, we have 2-3 thousand image stacks that we analyze, with the help of a couple of G5s. Those images are originally recorded on a Windoze machine, which tends to crash from time to time while acquiring. I think I first got into Macs due to the fact that even a 4 year old machine (which at the time was a Quicksilver G4) was stable, and able to crunch files, that a brand new P4 PC with 2 gigs of Ram was having issues running. From that standpoint, Macs simply get the job done, while other systems simply lag. In theory a Linux system has far fewer buffers, and the method by which it reads to memory is even more efficient, and thus would make an even better graphics machine. The lack of software, and the open source code mentality, however, seems to place it behind Macs in terms of graphics productivity.