Can someone please enlighten me as to the rules for selecting second card please.
Yes make sure you pick another hd4850 with same memory. Different manufacturer does not make any difference.
Both cards have to be running at same clock. Crossfire will render at whichever card has the lower clock setting.
You can also crossfire 4870 with 4850 but the second card will run at the slower hd4850 clock setting, thus dont waste your money on a card with higher clock settings since it will make no difference to you.