Or, should I start watching the CPU usage on my 2008 MP and base an upgrade on how much CPU I currently use? I've honestly not paid any attention to this and when I checked it just now I've got approximately 80% of my CPU idle.
IMO, you should check that before you made the decision.
If you want to check it, you may just do your normal work on the computer for 15 min, and the open terminal then type "sysctl -n vm.loadavg".
There will be 3 numbers displayed. They are the load average, which means how many cores demanded in your workflow. The 3 numbers corresponding to the demand of the last 1min, 5min, and 15min.
If any of that 3 numbers are >8, than means you can benefit from the Hex core CPU (12 threads). However, lets say the number never go above 8 (e.g. only 5.29). Then most likely you can't even fully utilise all 4 cores (8 threads), spend extra money for the Hex may be a waste.
Of course, you may repeat the same process with your most demanding job. Then it will tells if you can ever benefit from having more cores.