I can't answer your question about caster gear, I've been rather confused about that myself.
The caster level at which a spell caps usually depends on its level.
Level 1 spells mostly cap at caster level 5, 2-3 ones cap at 10, 4-6 ones cap at 15 and higher ones cap at 20.
There are exception of course, polar ray is one, and some spells, according to their description, don't cap at all.
Your best bet for checking spell descriptions is the arcane spell list from the wiki:
http://ddowiki.com/page/Category:Sor..._Wizard_spells