I couldn't get my GT240 and 6870 top play nice together, so only my 6870 mines. I get ~280Mhash/s with 1040 core and 300 mem. keep in mind my 6870 OCs really badly, 975 core in games = instant BSOD. Try using MSI afterburner
I get ~315Mhash with 5870 on stock frequencies. w/o the 8800gtx together they make on stock about ~345Mhash
so i dont let the 8800gtx to do nothing -not worth the electricity.
but they work well mining and gaming.