like Shishka said, depends on the game engine.
but some basic rules…
try to condense your textures, use as much of the 1024x1024 square that you can. poly count is important, but also placement is to too. most game engines wont render the polys unless you see it, so unless you have a huge outdoor environment, you should be fine. most game engines work with triangles, so convert your models to 3 sided faces. learn to bake your textures onto your models, that way, the game engine doesn’t have to render lights [well, not as many].
reply with what game engine your using and we can help with some more specific details.