Pool mining heavycoin with the CPU minerd has yielded about 52 coins in 24 hours (8 threads on an i7 3820 @ 3.6 GHz = 154 khash/s)...
I tried a bit. with 4 thread everything was ok, hovewer with 8, the gpu usage of the amd card (and/or the one on riser) was not stable at all . The gtx780ti didn't show this sort of behaviour.
I'm CPU mining while mining middlecoin.com with -i 0,0,0 -H 2,2,2 on a 780ti and (2) 660ti, no errors or problems and I'm even surfing the web while maxing out at 8 threads. Not even a slow down on my GPU hash rate.