Yeah, Kwag is probably right. When it comes to speed you will have to probably experiment a bit yourself. Otherwise here is what they say: (and "they" talk a lot, don't they..)
http://www.techbargains.com/hottips/hottip1/index.cfm
"Intel's Pentium 4 is their mainstream processor. It is a controversial design that executes fewer instructions per clock but focuses on maximizing CPU clock speeds. A Pentium 4 clocked at 1.3 Ghz runs business applications about the same speed as a Pentium III at 1 Ghz. The Pentium 4 is optimized for applications that utilize lots of streaming data such as Quake3, voice recognition, etc. "
"AMD's performance CPU is the Athlon XP. It offers very good performance at low prices. This CPU performs more instruction in a given clock cycle than the Pentium 4. An Athlon XP running at 1.67Ghz executes business applications close to the speed of a Pentium 4 2.2Ghz but at a lower cost. This CPU does use slightly more power than Intel's P4. (If you want to save on your energy bill, get a value processor) AMD prices their processors aggressively to compete with Intel. Avoid the older Athlon and stick to the XP as it will perform 0-10% faster than the non XP version. "
So it seems like for everyday use, P3 is pretty fast, and Athlon XP seems to be the best bang for the buck. However, for running a particular program such as TMPGenc or working on video, it seems like P4 was made specifically for that purpose and should perform better. Who knows...