if you crossfire 5870 with a 5850 the 5870 will be ''downgraded'' to the 5850 (clocks go lower). I'd recommend crossfiring 5850 witha nother 5850 so you don't waste money
Are you just looking to crossfire something ? You were just asking about 5770's ? I don't get this. You obviously don't have either of these cards right now from your other thread. So why would you purposely crossfire two different cards ?
Tell we what do you want? I mean your plan, are you looking card for CF? or buy to upgrade the old card?