...save your game before you enter and then reload that game multiple times, you will eventually find a ring or amulet that enhances every type of magic. I don't remember if you can find those in other magic shops and I don't think you can find +5 level items in shops but I'm not sure about that. If I remember correctly, items of a specific magic stack with the effects from the 3 artifacts for each 'class' of magic (light and dark - body, mind and spirit - elemental) but do not stack with each other. i.e. two rings of fire magic will not have more effect than one. Someone else will know that for sure.
The easiest way to see the effect of the increase is with the elemental artifact (Morgan) and a ring of water magic - then cast Lloyd's Beacon with and without various combinations of wearing one or the other or both and note the difference in number of days that it lasts.
