From what I understand the FireWire ones will do HD but until Apple releases the APIs in Tiger so that the drivers can be properly written the hardware requirements are horrendous. Right now the software developers don't have access to the APIs that they need to offload the video from the CPU the way the Apple apps do like their DVD player.
Tiger will be the first version of Mac OS X where the video APIs are completed. Not that I really understand it but
H.264 is going to bring some wonderful things from what I read.