For 5850 card, around 300-340 MH/s is good.
If you get 5870, you are going to get something about 370-400 MH/s.
Also, I would not recommend you to crossfire two different cards. Buy another 5850 and put it crossfire with your existing (or buy two 5870).
I'm pretty sure for mining you don't use crossfire at all. There's no frame to split in two and render separately on each card, so you lose performance. You want each card doing its own work to get max performance from each card.