why are game models done with tris?

[kikimora]

why whywhy??? don't game engines support n polys or something?

nope. At least most don't. The simple fact is that tri's are the simplest polygon and so reducing things to their simplest level makes it more efficient to process. If you know you are dealing with 3 sided objects, you can just forget about that part and move on. If you were dealong with n-gons, the first thing you would have to do is figure out how many sides it had which would involve counting the verts. Imagine a model made up of tris, quads, pentagons etc. It would tie up the processor just trying to figure out which faces were legal and then how to transform & shade them, then how to texture them etc etc.
That's before we touch on things like traingle stripping etc.
It's just easier for the engine this way.
I don't see why it's such a big deal for you anyway. Model how you like & then just convert to tri's. Not hard & some exporters even do it for you.

[kikimora]

thanks Mike i guess its not a big deal after all...I have been brainwashed by the whole npoly thing: "quads are the best yay for quads if you use tris you go to hell and die in teribble pain."

yeah that's a real snag with tri's - the whole going to hell & dying in terrible pain thing. I guess all game artists are doomed but I think we all knew that already anyway.
Seriously though, the only thing about quads is that they smooth better and more predictable than tri's really. For game models this obviously isn't even an issue. Even for high res models, the use of quads is by far secondary to the topology of the model, the edge flow and layout. Those are the more important concepts to consider by a long way, especially for animation. (look at kuman's model for an example of good flow even in a model for a static render - he doesn't worry too much about the quads/tri's issue, but you can't say his models suffer for it can you? (Big up James. ))
Even Bay Raitt uses tris from time to time & considers the flow of his edges to be of primary concern. He's more an advocate of the 'no poles' modelling philosophy, (anyone unfamiliar with these terms should check out his site & the forums over at spiraloid - I don't have a url handy but a search on his name & the name of the site should turn it up,) which concentrates on making sure no verts have more than 4 edges attached. The reason for this is that verts with poles often mean that an edge loop terminates abruptly or badly.
Flow is EVERYTHING in a model. (well give or take,) & quads are nicer if you can swing it.