If you trap a Golden Saint in a Soul Gem, you get a good cast capability. But I haven't yet found an item which can hold even half of that power.

Anybody found something?

Golden Saints are one of only two creatures whose soul lets you enchant items with a constant effect. The other is Ascended Sleeper. I have used GSs and ASs to enchant several items with constant effects:
Exquisite Pants and Skirt: Restore Health 2 pts each
Various rings, worth 120 and 240 gold enchanted with levitate, invisibility etc.
Exquisite belt: Light 20 pts (Might be more, 30 or 40?)
Glass Helm: Night Eye 10 pts

I think I used a GS to make a ring of Storm Atronach summoning, just so I could use it a lot. I would reccomend going with constant effect stuff though. The trick is finding the most expensive clothing, rings and amulets. Be aware you can only wear one amulet, two rings, but you can wear pants, a skirt and a robe all at the same time. As far as armor, I think the best for enchanting is ebony or daedric. I use light armor myself so I'm not sure. In general, the more expensive something is, the higher enhantment value it has.

That "power rating" you are refering to relates to the strength of the spell(s) you can place on the item. The charge you get from soul gems is just like a battery charge. There is one other attribute that relates to magic items and that is cost/use. When you have less charge than the cost/use you can't use the the effect. Note: you can recharge magic items by resting or using soul gems.

Golden Saint soul gems are best used for constant effect items.

Personally I just made some utility items to make my life easier without making the game too easy.

Shirt with permanent Bound Longbow and 8 points of Sanctuary.
Hot Pants - Exquisite pants with Fire Shield.
Shoes of Flying - Exquisite shoes with 1 point permanent levitation.
Shoes of Swimming - Exquisite shoes with permanent Water Breathing.
Glove of Vision - Extravagant glove with 20 points of Night Eye.

I did use the GS soul gem to make a cast when used amulete of summoning Golden Saints for 45 seconds. It basically served two purposes. One I could summon the Golden Saint then kill it while trapping it in a soul gem to make more permanent items without hunting down a Golden Saint. Two, when summoned during battle they allow you to retreat to a safe distance to heal or use ranged attacks.

Just my 2 cents,

Kermit