If you are going to go Flex atx, then I would just look into the shuttle barebones units. The newer models have an agp slot as well as PCI. Personally I would rather a micro atx in a coolermaster case or something.
It does have 1 PCI slot so you may be able to use a PCI Radeon. It really depends on your definition of a HTPC. This board is really meant more for the small screen ( < 40 inches?) or computer monitors. I am planning on trying one after the first of the year as a small unit for in the bedrooms to watch mainly video from my central server on SDTVs.
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