The landscape of graphics engines is always changing and with plenty of attention for Unity, it may be worth keeping a close eye on others like Unigine. It runs under windows and Linux thus far and seems to produce some pretty nice visuals.
[images of the Unigine engine via their website]
Check out their site for more pics and such - here are a few videos as well:
Oh and I was pointed the way of Unigine through Twitter - still enjoying myself on there...