alright so me and a friend is going to fool around making some low-poly models for a future game project, and I was wondering if anyone knows of some good guidelines for the no of polys for various model types... I realize that there are probably a million answers to this question that could all be right, but general guidelines would be nice.. I have no idea whether 5, 10 or 50,000 (in total) on the screen at once is possible on "normal" gaming pc's.

So anyone got wisdom to share in that area for:

buildings / terrain (it's going to be "outside")
minor characters, vehicles & monsters
major characters & vehicles

any info would be nice