You might get better responses to this question in the game engine forum.
People ask this question a lot, hoping for a clear-cut guideline that they can use, but that number can vary greatly.
There are several things that affect the framerate:
Number of vertices
Size and number of textures
Rendering Mode (GLSL/MultiTexture/Texture Face)
The number of vertices has an affect on speed, but it doesn’t contribute everything. On my computer (Nvidia GeForce 7200LE, Intel Core 2 Duo 2.8GHz) I can run over 100,000 vertices at 60fps easy, but on my game with only 1200 verts, It will often run at only 30fps because of textures and physics. Don’t get all your graphics done, then realize that they use all the resources, so none for physics.
The textures have a pretty big impact also. If you’re using GLSL, Normal maps can suck up a lot of speed pretty quickly, especially if you normal map everything.
Physics use a lot also, so don’t forget that they will take a lot of resources when making your graphics.
Logic doesn’t take up much, unless you’ve got some super-inefficient script.
This has a huge impact also. GLSL will sometimes take 10x more resources than say, MultiTexture, depending on the game.
So, to wrap this long post up, if you’re using GLSL and you have an environment about like a FPS shooter with one character, shoot for less than 2,000 verts, but use a normal map.
If it’s MultiTexture, you can probably go at least 5,000.
Basically, if you’re game is running slow, try to lower the vertices or lower the texture size or number of textures.
About subsurf: I try not to use it, but if you need to apply a level 1 just to get some more verts to work with, go for it.
I hope this was helpful.
triangulatin your mesh will increase speed quite a bit to.
Simply, a squared face has 4 sides, and a triangle face has three sides.
In organic modeling, or smooth objects you want to use quads(4 sided face) If it’s rigged, they deform better. If you’re sculpting, they deform better, and it’s easier to keep good edge flow.
For objects without many curves, you can get away with triangles.
I suggest checking out one of these tutorials: http://www.blendercookie.com/?s=topology+review