You'd get a lot more rendering-speed "bang for the buck" by buying a $299 Dell special and setting it up as a network rendering station; might speed up your overall renders by about 33% or more.
encoding is a real dog. i rendered a 2 hour clip uncompressed in vegas on a dual xeon 3.4, with 2GB ram on SATA drives. then rerendered with CCE and the whole process took about 70 hours and took up around 230 GB of space. adding ram did not help a bit.