FPS average

That depends on so many things. I have a project that has run from 1000 fps (on my pc) to 11 fps (on my school''s computer). What you usually aim for is at least 24 fps. Any less and it looks choppy.

i would try to target for 30 frames per second in medium to somewhat heavy action scenes. the low end pc should get this type of framrate with turning most options down. for a higher end pc, i would suggest approx target of 100 frames per second with all features turned up to max. the reason for this is simple because most ppl rather play somthing that is smooth vs something that looks better. 30 frames per second is technically a low framerate for smooth motion on a pc, but is generally acceptable to ppl who have no choice. the reason 20 frames per second is considered low is due to the fact that games dont employ motion blur, thus the eye has trouble with the animation, and tends to see it being somewhat choppy. movies on the other hand store all the light info thus perserve motion blur cues to help give a smoother look.

though its best to have some friends play the game and ask them if its too choppy. ussually you can feel when the control is unresponsive due to low framerates (another reason why many serious deathmatch players tend to use lower resolution and quality settings to keep the framerate at 60 or higher). if your game is scalable then its not much of a problem since the gamer could lower the detail settings for better speed. take care though that you dont allow required gameplay details to be sacraficed. 2d games ussually are not scalable, but 3d games can be though it requires some work. either creating multiple versions of models with different amounts of polygons, or actually scaling the poly count using a LOD algo (in realtime or at loadtime). levels on the other hand cant really be scaled too much with the exception of vertex lighting instead of lightmaps.

in short, you idea of 40-50 is about right.