If you can afford it, dual Opterons. Dual Xeons would probably be better than a single A64 chip... but again more pricey. The PPC970 is also very good at video editing.
what it really comes down to is you get what you pay for
you can have a decent system with an Athlon XP-M 2400+ OCed to 2400MHz and have 2 gigs of PC3200 RAM in there, and itll cost you 75 bucks for the proc and 300 each for each 1GB RAM module, or you can go for an Apple G5, or you can go for a dual Opteron 244 with 2 or more gigs of RAM. the more expensive it is the more powerful it will be
I say, AMD 64FX-51 or 53 over any Intel cause its a much cleaner process, than Intel's chips. But if you wanna wait untill winter, the P4 prescott is getting 64-bit extentions. But right now I would go with AMD, or Apple's G5.
Go with a good Pentium 4 Northwood. The Pentium 4 will be alot better for encodeing with Hyper Threading. Alot of the A64 motherboards have poor memory compatibility and wont hardly even support more then 1GB.
A Pentium 4 outperforms even dual Opteron 240's in encodeing. Unless you want to fork out alot of money on 246's or 248's then go with a Hyper Threading enabled Pentium 4. They are the best option for the money.