Two 4870's will work together they dont need to be the same make/model. Usually if there is a differance in clocks then the higher clocked card will down clock to match the lower one. people have reported both cards running at the higher clocks but that is really quite rare.
Any memory differance will again be a lowest denominator issue. If you put a 512mb card with a 1gb card you still only have efectivly 512mb of memory for the video as both cards need a copy of the same info and the extra 512 on the 1gb card cant be used as the limit is the 512 on the other card.