It depends on they resolution that you are playing at. The higher the resolution, the more of a difference you should see between a single HD 5780 and XFired HD 5780.
If you have Crysis, then use that compare performance differences. Enter the following command in the console to display your frame rates:
r_displayinfo = 1
You can also download a program called FRAPs to get an idea of your performance, however, FRAPs is somewhat demanding so I think you will take a 10% - 15% performance hit when using it, but you should still be able to tell the performance difference with XFired HD 5870s.