I installed and configured the SMP client this evening and it's currently running through its first data set. With all four CPU cores @ 100% utilisation the system is still perfectly usable and CPU temp has only risen 7C above idle level. I'll run the GPU client alongside it overnight and look at the logs in the morning to see how they play together... may have to back off the "CPU Usage" slightly on the SMP client to avoid throttling the GPU one.
Already thinking about resurrecting my old Athlon X2 purely as a folding rig. We might get a few weeks work out of my 8800GT GPU before it finally croaks (and I can contact BFG about a warranty replacement!)