Price Performance ratios are better with AMD, but with the new Pentium 4A's, that difference is narrow. If going for the P4, make sure you get the P4 A, and I would suggest an Intel 845 Chipset (With DDR Ram) over Rambus memory.. Though thenically, Rambus is probably faster, I don't count on it being widely availalbe and cheap 2 yrs from now.. (DDR is soon to be the de-facto standard for PC memory). If using AMD single CPU, get VIA KT333 chipset Mobo for best performance. With AMD CPU and chipset, it's possible to get dual CPU system. (
www.tyan.com).
For Video and Audio editing, if you want to increase performance and don't mind the price, get SCSI controller (Adaptec?) and a couple Seagate Cheetah's 10 000 RPM drives... Use Linux software RAID to split the load evenly between the two drives (I forget what RAID level that is, see the Linux RAID Howto). That will effectively double your througput. (Keeping in mind that there is no hardware failure reduncy this way). If you do decide to go this route, mount the hard drives at least 1 drive bay appart with fans in between and around... You won't believe how hot these get. (approx as hot as the old Pentium 100's used to get without cooling). Better yet, put them in an extertal SCSI hard drive case.
Also, for video and audio editing, give consideration to Mac OS X..I have no experience with it, and certainly don't expect to get as good a configuration as I outlined above for performance, but the convenience of software availability might well be worth it, and you still have *nix running underneath it all.