If I could do it again, I would have purchased a Mini instead of the ATV. The ATV is too crippled to me. I have it hooked up to my projector. I don't rent movies from the iTunes music store, I rarely stream movies from my Mac. I have Boxee installed on the ATV, but it can easily be installed on a Mini. I would rather have the functionality of a full OSX machine rather than a crippled interface of the ATV.
As far as HDMI, just use a DVI to HDMI coverter. I have a drawer full of them. diNovo Mini from Logitech and you are set.
I'm sure there are many many threads about this same subject. Have a look around.