Year ago it wasn't possible to use more than 4. I've seen some reports about working 3x5970 configs since then but under Linux.
What's about Windows? Was 4x limit been increased finally?
One of our users trying to setup 3x5970 system and even with dummy plugs/monitors attached to each of 5970 DVI ports it fails to operate properly -- system simply lock ups after several seconds of work. There no problem with PSU, so this isn't the case.