anomad - it turns out to only have one usb socket and 2 firewire expresscard adaptor. (Card is Belkin Part no. FSU506).
Ok i've spent a couple of hours messing about with 4 video glide dc60+ usb capture units and a 2006 macbook pro.
1 USB Input - superb quality image, tested capture size up to 2048x1536 - no problems
2 USB inputs - Works fine on both units , maximum capture size 360x288 on both.
(reduced usb bandwidth setting to 50%)
3 USB inputs - Works fine on all units, maximum capture size 360x288 on all
(reduced usb bandwidth setting to 33%)
4 Input (3 USB & 1 Expresscard USB adaptor) - Will only display 3 devices at once.
(tried many different combinations of capture size/usb bandwidth settings)
One of the 4 screens turns purple when not working, if another working device is unplugged the previously purple screen unit starts working, it does this when unplugging any input, usb or expresscard.
4 Input (3 usb & 1 firewire) - Same result as above.
4 input - (2 usb, 2 expresscard (using a powered usb hub to give enough sockets) - same results as above.