What is a for today typical polygon count that most PCs can handle
I would say according to current generation consoles -ps3/xbox360- and not counting next-gen it's usually around 10k for the main characters which is reasonable taking into account quality/performance in the current gen. For enemies it's around 2k-5k, depending on how important they are. Given that PS3/XBOX 360 are 7 years old** I can safely say most today's PCs can handle that.
The problem though
Polygon count is a tangible number (and FPS per se) yet alone is not really an accurate or even good indication of the overall game performance. There is much more in your game than poly count; fillrate, draw batches, fragment shaders, rendering passes, culling..etc the best way is to profile your application and find your bottleneck. Real time rendering is a great book for different visual appearance and optimization techniques it even has a full chapter dedicated for pipeline optimization
**I didn't take into account that PS3/XBOX 360 are optimized for rendering/games.
I am thinking about having animated 3d models in the background for
atmosphere, like spectators.
Well, depending on your game; but the question is, why use fully detailed 3D models while they may not really add much to the game because they are not actually at the very center?
Most games don't use real 3D models for spectator look at Fifa for example, they use 2d images without losing much of the atmosphere.