Forget the G5s the MAC PROs will be out next month. Other than that it depends on what you want to do. If you want to go pro Avid and Final Cut are the way to go. If not then download some demos and find out which software you like. If you decide to go with a windose machine then the new Core 2 Duo chips from intel are stomping on AMDs fastest right now. Also the competition between them has created a price war which makes it a buyers market for chips right now just go at least dual or quad core.(quad is a very expensive option and requires workstation class chips and mother boards) Other than that a gig or two of your favorite ram, as much storage as you can afford and a decent video card and you got yourself one bad ass system. If you want exact specifics for every cable and screw you need there are many sites offering tutorials on how to make your own box.
