Don't X Fire / SLI low end cards. In most instances a single GPU setup is better or equal. Dual 7770's performance is somewhere in between the 7850 & 7870 but if you're resolution is more than 1920x1080 a single 7850 is better. The 7770's can't push the frames at higher resolutions.
A single card will also use less power & produce less heat in your case.
thanks for the info