Generally, just hover your mouse over the "Action Points" entry in the inventory, it will give a complete rundown of how much beginning AP you get from speed and perception, how many additional AP per turn from speed (and glass cannon) and what's your max AP based on constitution...
Which you should invest in depends what you want to improve.
Major first strike; Perception (also useful for secrets and traps, so definitely one char should get this)
Lots of AP per turn; speed (or using the GC-tactic floating around, GC and con)
High AP Max; Constitution. Normally this really never is an issue, except with Glass Cannon you might get more AP than your max is, and that's just a waste. Also if Max=Turn AP any leftovers go to waste, so then it would be wise to improve it.
Hopefully this isn't too confusing...
