thead has a lot of info from upgraded G4s. You're looking at 20 tracks with 5 plug-ins per track at 24bit with 128 buffer on a Graphite G4 with a 1 Ghz cpu upgrade. The Graphite G4 has a slower motherboard than the more recent models, 100Mhz system bus instead of 167Mhz, 2 x AGP instead of 4 x AGP (probably makes some difference from a throughput point of view) and slower PCI slots. I'd guess that means if you were getting 32 tracks + 10 auxes with 4 plug-ins per track that a typical mac is going to be about 40% slower than you're PC but at least if you'll have midi patchname support and lots of cool plug-ins you can't get for windows. With Protools 6.0 being multithreaded I'm sure a dual G4 from 800Mhz or above will at least match the performance of you're PC if not beat it but don't quote me on that. No one really knows for sure exactly how much faster Protools 6 will be until it's out.