Brand new i7 2600K CPU, no huge increase in Encoding times over old athlon 4200+?

One thing would be whether Camtasia would support multiple cores - but I think the disks are the real bottlenecks. Any XP version would be at best a single socket operating system, and would have trouble utilizing more than at most one more core. You would likely have greater success with any version of Windows 7 - and new disks.