How is it determined if your target is crippled or not when you get a critical hit? Is it the same chance on any critical, or is it based on how much damage the critical does? Also, will a critical hit do more damage if it hits for instance the torso instead of an arm?
EDIT: On a side note, why is there an option to make an aimed shot on the torso when according to the wiki you don't get any bonus critical, can't cripple the target and pay 1 extra AP for it? What am I missing?
This has probably been covered elsewhere in better detail, probably in the Wiki.
There are two different rolls. The first determines whether or not the hit is critical, and is Luck/Critical Chance dependent (among other factors). IF that hit is critical it rolls a second time to determine the severity of damage and any additional effects such as bypass, knockdown/out, and cripples. The second roll is affected by Better Criticals perk (adds 20 to that roll), as well as armor modifiers of the target, etc.
No idea about the aimed torso shot, I don't know anyone who uses it.