Hey, im wondering if i should use $200 to upgrade to a 4890/4870 or just keep them? If the performance increase will be less than 50%, please let me know.
2x 3870s in Xfire performs equal to or slightly better than a 4870.
You have a fairly decent set up and I'd suggest you wait for the prices of the 5x00 series to go down (prolly when the GT300 releases) before you upgrade.
Anyway, I know that the 4800 is a great improvement from the 3870, but I am with the others who say to wait a while. More competition over time will most likely drive the prices down a bit across the board.