Do you have the board and PSU for it?
As for if you should that depends on cost. If you can get them cheap then go for it. Even if you got them for $100ea however I'm not sure I would. You could get a new 7850 for that, a 7870 if you pony up a bit more. Those would be better as they have more Vram and use less power. The 5850s might be a bit faster but you also wouldn't have any CF issues. (MS, lack of drivers for new games, etc.)