ddr5 and ddr3 will not change the crossfire with the 6670. if you have the ddr5 the system tones it down to ddr3. save yourself some money and get the ddr3 version.
Get the one i sent you. its the best one on there.
That's not true. The idea of crossfire and SLI adjusting it's clocks and speeds to the slower card is a myth. Testing shows this to be wrong every time (though I haven't tested this one on an APU crossfire setup), but I agree with your suggestion of getting the cheaper card.