how much spell penetration do I need if I am an arcane mage for pve. and how good is it, am I better off getting more crit or dps?  thanks

each point of spell penetration rduces targets resistance by 1. not sure how much is needed but i dont think very many actually get gear with penetration

Penetration is for PvP.

The way I understand it is, every character has a chance to resist something(even if they have no +resist). This increases if you are more levels higher than them. 70vs65 has more chance to resist than 70vs69.
"+spell hit" increases your chance to hit them in this respect.

They can get an extra chance to resist spells if they have +resist(maybe +fire resist, +shadow resist). You can get this from BUFFS or GEAR. This is ARITIFICAL resist and not NATURAL resist from differences in level.
This is what +spell penetration is used for.

Therefore penetration is used for PvP and spell hit is used for PvE because PvE bosses don't have +resist(as far as I know) and only resist you because they are higher level than you.
However, players may have +resist BUFFS or GEAR which will allow them this EXTRA resist.

Not too knowledgable about the mathematics of the game, but that's how I understand it anyway.


Oh, and if you're an arcane mage casting arcane spells you need around +70 spell hit.