LFG 4 Composite capture card work fine in all intel MacPros. (There's also one for a G5 if you really need it...)

It is possible to install 3 x cards in one mac and therefore run 12 inputs. The 'release' version of catalyst only supports 8. But custom versions support more.

Full Active Silicon range is available from me and other distributors

Let me know if you want to try one out before making a purchase

Simon