Do we talk about realtime animation or prerendered graphics (you mentioned both)?
Poser would be about prerendered art, because as said before, the characters have too many polys to get decent speed. I would say to use Poser is a bad idea. Its so favourite because you just need a small amout of knowledge to get results. But first of all, you can recognize that ugly "poser look" without much effort, and it's not so easy to customise your model in every way. The interface is not so good when comparing to other established 3d apps. If you just need a generic human model for prerendered animations it would be enough though. Perhaps you could edit the renderes frames in your favourite graphic app to get away from the poser style.
Milkshape is the choice for wintermute realtime character animation, because afaik thats the only supported format. Its cheap, easy to use and quite powerful for generating realtime content. It wouldnt qualify for prerenderes graphics, because it lacks an powerful render engine.
I dont know much about animation possibilities in Blender, so maybe thats an alternative.
Of course there are the big, expensive 3d apps like 3dsmax (+character studio), Maya, Lightwave, Cinema4d and so on. Character rigging, the setup for animations, is not that easy although there are lots of tutorials everywhere. Powerful with a learning curve, and expensive to buy.
As you said, you could use modification tools of good games, a nice idea. I dont know if you will get animation tools though. For example, Max Payne 2 uses 3dsmax for creating animations even camera paths).
Zbrush 2 is also a nice application and features Zspheres, so you could eays pose your model and create an animation of single frames (much easier to learn character rigging). And ZBrush isnt that expensive, too (but more expensive than Milkshape
).