Well its a balance between processing power / how much time you want to devote to solving your intelligence. There are also logistical things, such as what do I need the bots to do (fill a gap for players or something more complicated). You have to deal with an extremely complex set of decision trees. 3D Navigation is very difficult to calculate an optimal solution for path planning purposes because of the sheer number of possibilities....
Basically a tremendous amount of probability, binary searching (decision trees), activity logic, path planning, knowledge of the mission state and objectives, cost functions... etc etc.
I lot goes into AI besides telling a bot to move to point x,y and fire 3 shots. I suggest you pickup a book on the subject, there are a lot of good ones out there, but be prepared to open your math mind
The Valve Developer Wiki has a whole section devoted to AI:
http://developer.valvesoftware.com/wiki/Category:AII want gun game
This is already out in Beta 3.1 and there are two servers hosting it 24/7