No , you will not put a vid card in this build if your using the same case . I was able to mod in a Ceton tv tuner but it was a PITA and is very tight . There is no toslink either.
I absolutely love this htpc for it's size and the " big boy" punch it packs . Not that it matters much , if any but the DH61AG does not come with sata 6g it is purely 3g. I doubt you would know the difference as you would be hard pressed to saturate 3g.
You would (will) not be disappointed in this unit
I cannot speak for anything to do with Linux ... sorry