Single 6770 card produces ~220MH/s so about ~0.022btc per day per card. That's my guess. :p
Anywhere from 450-500Mhash/s with RPCminer and some modifications to the memory powering on the cards. Plan on under-powering the cards memory and clocking the GPUs a bit. Word on the street is they run way cooler than other cards even with clock modifications.
I've heard of 400Mhash/s without tweaking while running two of these cards side by side.
Sources:
https://en.bitcoin.it/wiki/Mining_Hardware_Comparison