Just make sure you have a passkey and keep a core free for each gpu you have. Oh and don't bother folding on the cpu, its not really worth it.
GTX660 looks like it might need older drivers to get the best performance (roughly 2x current drivers) Try using drivers 327.23
390's looks like they should be roughly 3x faster than the GTX660's
I put a passkey in and my PPD is rocketing lol... Thanks!
Also I put the old drivers on the other machine, FAHControl is stuck on "READY" with the status "Waiting On FahCore Run" - Any ideas?
I think its to do with the way the slots are configured. I removed and re-added them, same thing. I also started playing with the index slots and managed to get them to run momentarily and then stop again with an error popup. If I leave the error on-screen without clicking 'OK' it stays at "running" and calculates the PPD, but the WU's don't seem to progress.
Currently just re-installing the client to see if that helps.