Im mining with 4 X 7850 / 2 x 7770 / 2x 7950
Makes about 3000 KHashs consuming 1,5 KW/h :-D
Im payinf 0,28 $ per Kw/h, so at the moment its really worth the effort.
What types of chassis do you use for those machines (and perhaps in particular, the quad-GPU one)?
I have a well-ventilated SFF case for my single-GPU system, but I guess I'll have to use milk crates or something when it reaches 4..