I'm hoping to get GTX 760 second hand for around $210 and SLI'd with my gaming rig (I get ~310kh/s with my 760) but the power usage won't decrease much from 5850+6670 combo
I believe most have said you cannot 'SLI' two cards together and mine with them, so may want to work that into the equation or check into it further.
I believe the computer only sees SLI'd cards as a single unit, so only the first one would mine. I haven't done it or even tried, but I swear I read that someplace.
Good luck!