I have a level 40 'mage', and I'm in full plate armour (as Dungeon Master said) - you just need to focus on pieces that have the right enchantments (+int, +willpower, +magic damage, +mana regen and some +healing is also useful).
You should concentrate on int (improves mana regen, magic damage and magic resistance) - I found that keeping my willpower to no more than 50% of my int was more than enoguh. In fact, now I have int of 100 (including equipment) my mana never goes down, even on continouos level 5 fire-breath, so the size of my mana pool is no longer relevant.
I used a bow at lower levels, but didn't put any points into bow skills. It was only when I got to level 15-20 that I could fight by magic alone - but it's worth the effort, a DPS mage does unbelievable damage. Right now my fireballs (15/15) do up to 4,000 damage in a 12m radius, and I can launch 7 magic missiles (also 15/15) per 1.5 seconds for a damage of around 400 points per missile. And that's after they nerfed magic damage in the patch (I could kill all the bosses in one shot before the patch).
As for skills - Magic Missile is a must, as it's the only spammable magic attack. Some people recommend maqic blast, but I found fireball more useful (even after I finally got magic blast up to the same level) - fireball is an AoE, whereas magic blast is single target and only really useful against the tougher bosses. Desctuction is also awesome, as it improves your magic damage much faster than adding int does. At least one point in healing is also highly recommended, as you probably won't have much health. I found firewall useless, as it was too easy for the monsters to avoid the flames.
I use a mace and shield - I don't fight melee, but they seemed to have the best magic enchantments, and allowed me maximum flexibility when it came to charms and enchantments. Using one of your precious malachite gems to get the +100 magic damage enchantment on your weapon is recommended (the damage is added to your spells), especially since they recduced level 9 damage enchantments from 70 to 50 in the patch. I used the rest of the gems on level 10 health regen on my jewellery - each enchantment is equivalent to 0.25% regen per second, which is the equvalent of 50 strength points (two strength points only improves your health regen by 0.01% per second, so don't waste points on strength).
By the way, if you get mindread up to 15/15, it uses no XP, and you can just mindread everyone. You'll be amazed how many extra skill points you'll get from random mindreads, more than enough to make up for the points spent.