You'll find PvE bosses sit on 18'200 resistances, so this is the target for penetration.
A Tank brings around 11'200 through major and minor breach, and crusher enchantment.
This means you want to find approximately 7k for most situations - however in organised Trials it drops to almost nothing.
33'200 Resistance is 50%, this is a hard cap.
Critical chance has a hard cap of 100% I believe, because I don't think there are ways for the enemy to reduce this number.
Critical damage is hard capped at 125%.
Obviously in PvP their penetration will reduce your resistances, so if you ran 50k resistances somehow, you might have 50% damage reduction against all but a very slim minority who somehow get 20k penetration (I don't recommend it, just pointing out the feasability).