I went from a single ATI HD 4870 to twin ATI HD4850's.
I mainly did it because they were cheap, and I run Boinc so now I can run 6 units at once instead of 3.
A handy and neat side effect was getting twin monitors... very handy indeed. Just click and throw windows to the other monitor.
So now I have a 19 inch wide screen and a 17" regular LCD. Soon to be twin 24 inchers...