840 isnt too bad really.

I was getting ~320000 with any -f from 1->30 but cpu was spiraling out of control. I changed it to -f40 and now load is a steady .4 with a hash rate of ~318000. Things seem much more stable like this.

I also watched gpu temp and fan speed closely for a while. GPU temp was flucuating from 89c->92c with a fan speed of 40%->55%. I ended up setting it to 65% and GPU temps have leveled at 75C->76.5c.. with a 1c rise in 1 hour. Not sure what the best speed it going to be, but 90c is a little to hot for my liking.