did a Crossfire benchmark with the 7750 & 7770. The 7850 & 7870 aren't listed but it appears that's where the performance would be, between the 7850 & 7870.
The problem is extra heat (really not all that much), extra power draw (having the PSU to back it up) and getting the drivers to work without hiccups.
Edit: The better option is get a 7850 2gb or 7870 and sell your 7770. Newegg has a 7850 2gb for $150 and the 7870 is $195 after rebates.