Hey... you can get an Avalon unit to do 720 GH/s. It's totally easy. Just put 12 BFL singles in the case. Just a bit more wattage than a 4 card Avalon and you'll have room to spare AND you'll get almost 3/4 of a TH out of an Avalon.
You could even re-use the power supply and the control board!
Puerto, you're right. I was wrong. The Avalon is modular enough that you can throw out or sell those Avalon chips and put BFL cards in them! That is a killer feature.