If you mainly do encoding, then I would recommend a dual Xeon at this time.
Don't get me wrong, I really like the AMD64 by now, but for some things Intels are very competitive. If you have a look at the prices you'll see that you can get the 2.8 GHz 90nm Xeons pretty cheap, and the mainboards are less expensive, too, at least if you want PCI-X (not PCIe). Plus the Athlon requires registered RAM. Not only is registered RAM more expensive, it is harder to sell later and harder to reuse in other computers of yours.
Whether your encoder is multithreaded is had to answer as long as you don't tell us which program you use
At work I just got a dual Opteron and have two dual Xeons (130nm, though). The Opteron blow the P4s away for many things, but encoding and floating point is not one of them. Plus it was mighty expensive.
This is the mainboard I was eyeing for a Xeon system, $335. Newegg also carries refurbished one for $80 less.
http://www.newegg.com/app/View...=13-129-131&depa=0
This is the 2.8 GHz Nocona Xeon, it only costs $239 which is a snap IMHO.
http://www.newegg.com/app/View...=19-117-025&depa=0
If your encoding software is multithreaded and uses SSE3, then two of these hyperthreaded will blow anything else in the same price range away.