If you're looking for a solution that would allow for additional expansion via internal cards, check out the e-350 boards that follow the mATX form factor.
Does that x16 PCIe slot work with the Ceton InfiniTV 4 cablecard tuner?
I doubt it. I'm waiting on some parts to arrive to test the board, but the manual states the PCIe slot is only for a video card. It may very well be that anything occupying the slot will bypass the onboard graphics for the APU. I don't personally have a PCIe tuner, but I'll ask around to see if something is available.
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