You will be looking at Thunderbolt for your IO connetions. No current IO boxes use Firewire or USB.
IMHO the best IO device is the AJA IoXT. SDI in and out, HDMI in and out. Analog out if you need that. And it has Thunderbolt loop through, meaning you can then attach a thunderbolt monitor to the device, or a thunderbolt hard drive. Currently it is the only Thunderbolt IO that has loop through. And AJA hardware is dependable...stable, and always seems to be on top of updates.
If you need output only...meaning no need to capture, then look at the AJA T-TAP. Thunderbolt connection, and powered by that connection, then HDMI out and SDI out, with 8 channels of embedded audio. Small and inexpensive. I reviewed that on my blog at LFHD.net. Both devices work with FCP 7, FCP-X, Avid MC 6 and Adobe CS6