Be aware that Crossfire provides usually no more than 75% improved performance at the best of times and often no more than 40% or so (it really depends on the games).
It's also important to have a CPU that can handle the added card. To test this:
Hit CTRL-ALT-DEL, then leave the Task Manager running. Change settings to lowest update frequency. Now open a game and play for several minutes.
Leave the game and note your CPU history.
If your CPU is regularly going above 70% then I can't recommend a second card for Crossfire. (it's difficult to estimate now that we have multi-core setups as the game might not support all cores. You may only be able to get one core to 100% by using a better card or you may be able to get two cores fully and two cores partially.)
There are ways to make the game perform much better (better FPS). Anti-Aliasing is one way. Instead of 8xAA go with 4x. Turn down shadows. Experiment. Run FRAPS at the same time or better yet use a benchmark tool like Crysis has.
The higher the resolution of the monitor the LESS the need for AA (which eliminates jagged edges). A monitor like the new 27" U2711 with a resolution of 2650x1440 probably needs no AA and freeing up that extra horsepower can help compensate for the added resolution.
There's lots of tips for getting the most out of your computer for games.