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).
No Crossfire here enabled
5850 maxes out around 300 and something (775Mhz GPU and 300Mhz RAM)
5870 maxes out around 400 (895 GPU and 300Mhz RAM)
I also have a poor 5570 laying around maybe I will just plug it in :-)