Then you want a SageTV HD Theater combined with PlayOn.
It's also unlikely that the Roku has enough juice to run Boxee anyway. I'm not 100% on this, but I think it uses the same Sigma decoder chip that the Popcorn Hour and all the other media streamers use. You'd see Boxee on the PS3 before any of these devices.
On a side note, my experience with PlayOn was very poor. I'm surprised they're even able to charge for the software.