mixing ASIC cores on crossfire?

I am thinking about going crossfire this summer. I already bought a Diamond 4870 1gb card. I noticed on the manufacturers site that the ASIC cores are different from the 4870 to the 4870x2. The 4870 has a 770 core and the 4870x2 has 2x700 cores. I have seen on a chart here somewhere that they are compatible for crossfire, but wonder if there is any advantage to using the same card?

They are exactly the same, and the HD4870 will just underclock to the same as the HD4870X2, or just overclock the X2. Anyway IMO I would wait for 5XXX series ; )