GDDR2 is significantly slower than GDDR5, but that might not even matter considering the 5570 is such a weak card. The 4890 will KILL the 5570 performance-wise. Sapphire vapor-x is just a nice cooler on the card made by one of the manufacturers. Vapor-x doesn't matter.
But really, the 4890 is an awesome card, don't even think about trading it for a 5570. The 5570 is less than half as powerful as a 4890, and DX11 really isn't very important, at least in the next few years.