learn to search, http://www.gamedev.net/community/forums/topic.asp?topic_id=354455
the best method is to model a basic tree and use a plane with an apha texture on it. this is not the same as billboarding, it just allows you to fit 10-15 leaves per poly.

not really, just model a tree as you usually would but then use a multi-res modifier or something to lower the poly count. another method is to use alpha maps to make tree trunks and branches.

The number of triangles needed to make a tree look nice depends on the type of the game (e.g. FPS, RTS) as well as on the overall style (e.g. realistic, comic) and the quality of the others models.

And of course, LOD is your best friend.

yeah, LOD systems are sweet. they dynamically lower the poly count of an object depending on your distance, only drawback is more mem usage but that is minimal.