H.264 (Otherwise known as AVC), the new video codec in QuickTime 7 looks... really... good. The file sizes are smaller than regular mpeg4 or sorenson, and the picture looks so much better that there's really no contest. If you've got QuickTime 7 installed and you have a reasonably stacked G5, take a look at some trailers for really bad movies in HD.
Try resizing them. Yesterday I saw one of these trailers stretched across two 30 inch displays in extended desktop mode, no slowdown, no visible pixelization or artifacts... do you hear me? NONE.