At 1920x1080 you could see some benefit in newer game titles, or DX11 based titles if you like to max your graphic settings. Typically though, most game titles in DX9/10 mode are going to run very well at that resolution with even a single ATI 5850.
I only bring up Metro 2033 because it's sort of today's "Crysis" in that it's very, very demanding. It's one example of a game that really can play better with a Crossfire setup.
Assumign you're playing new demanding games with all the settings cranked up, Crossfire with an ATI 5850 could have its place. And your system is certainly capable of handling a pair of 5850's.
Just remember, if your monitor has a max refresh rate of 60hz, you can only use 60FPS.
Anything above that is for gloating/bragging only.