Under the beginner's guide I have this information in a spoiler under "Enhance Magic". The link to the guide is here: http://forums.taleworlds.com/index.php/topic,276440.0.html
But I'll go ahead and post the question's answer right here for a quick look. You might want to keep the guide open as you start playing the mod as it should be able to answer most of your questions
. Have fun and good luck out there!
Armor Magic Power Bonuses:
Twilight Armor = 0.5
Novice Robe = 1
Elven Mail Shirt= 1
Mithril Maille = 1
Apprentice Robe = 2
Vedmak Armor = 2
Journeyman Robe & Dark Robe= 4
Djanni Mail Shirt= 5
Adept Robe & Night Robe= 6
Balrog Body= 6
Expert Robe & Dread Robe = 8
Master Robe & Roid Robe = 12
Archmage Robe= 16
Hats and Helmets
Novice Hat, Novice Hood, Twilight Helm, Death Knight Helm = 0.5
Apprentice Hat = 1
Journeyman Hat, Dark Hood = 3
Adept hat, Balrog head, Night hood = 6
Expert Hat, Expert Hood, Dread Hood = 9
Master Hat, Master Hood, Void Hood = 12
Archmage Hat, Archmage Hood = 15
Staff Bonuses
Gnarled Staff= 0.5
Journeyman Staff= 1
Adept Staff, Shaman Staff = 2
Expert Staff = 3
Necromancer's Staff, Master Staff = 4
Archmage Staff = 6
Each of these numbers adds itself proportionately to the magic power skill score on every spell formula.
If any other armor and helmets are worn (ones that are not enhance magic) the score, to either the robe factor or the hat factor, will become 0.
If by adding both robe and hat factor the result is 0, there will be a -5 penalty to the multiplier *Multiplier being one of the numbers that helps determine your spell damage by "amplifying" the power of the spell*, so in theory you could wear metal armor and a wizard's hat or a wizard's robe and a metal helmet, but the bonuses you'll get will be significantly lower. If you get the -5 penalty, the spell's total magic power will be seriously lowered and could make some spells nearly useless.
For the time being there are no magic enhancing boots or gauntlets, but that could change in the near future. Currently these do not factor into the magic damage multiplier mentioned before.
So to Answer quickly:
Try to get at least a hat or robe. Even one of the lower tier ones will at least stop you from getting the -5 penalty. You only need to have
ONE enhance magic piece to avoid the penalties, but always keep in mind that if you were to use two pieces of enhance magic gear you could be dealing quite a bit more in terms of damage, especially with AoE's. Even one or two extra damage on an AoE spell can translate to +80 overall because of the amount of people you hit. Higher level enhance magic gear pieces can make the difference between night and die when it comes to magic power.