There are pros and cons to both its up to you to figure out which setup is best. It does improve your performance but only if its supported by the software.
I have a 40" viewable Wide Screen TV that i am running Full 1920-1080P HD video on for my monitor and From what I hear it makes a difference at this resolution at this size.
My two personal big issues with this set up are
1. the support for programs is not always available but this is changing i think with a lot of very new Games that i have been buying have this support and the games run phenomenal with it.
2. HEAT. It has been my experience with this rig since its building that by having the two cards inside of this medium size case has added extra heat and used up more room for air flow. If you had a Huge case with more then enough room to leave space between the cards this might not be an issue.
I think that all the newer cards are running hotter and hotter as they get bigger and bigger GPU's on one card but For my next build i think i will go for one Big card in a very large case so that i can maximize the air flow and not have to worry about compatible software.
This said I can not fault the performance at all. Every game with proper crossfire support i have run on this machine FLYS and looks Amazing with every bell and whistle turned on
Good luck
oh and ps....... Dont use the Crossfire/SLI performance of IL2 COD as a marker for weather or not you go with that set up. The support is still not working correctly so there is no way to know yet how this game will run yet when it is correctly fixed and supported.
Cheers