What kind of I/O do you need? Do you do a lot of H.264 encoding?
You should check out the I/O Devices from Matrox...
http://www.matrox.com/video/en/solutions/devices/
I have the MXO2 and love it! It will work with your MacPro and a MacBook Pro. You can get it with their new MAX H.264 Encoding Engine which will give you faster then realtime H.264 file creation. I saw a demo at NAB and was blown away with the speed along with everyone else around me.
You can see more info here..
http://www.matrox.com/video/en/solutions/encoding/
If you have any questions let me know.
Greg