I too find this issue daily.  As hard as it seems, the larger, faster computer should be doing the heavy lifting.  Save the older macs to do your daily duties.  Buying an expensive Mac Pro just for video or calculation work and then using lets say a G4 to use for everything else is tough to do.  I would want to use the Pro for everything.  Its not our old macs that should be taking server, encoding work, its the new ones, lol.  The best scenario seems to be a workstation for big jobs and a laptop for normal uses.  In the end, having another Mac to work on while another is busy is the way to go, no matter what you can afford.