With enchanting, the only way you can get a constant effect enchanted item is to use a grand soul gem with either a golden saint or ascended sleeper.
The gems by themselves aren't worth a whole lot. The value comes from putting souls inside of them. All creatures have a value assigned to their souls. Each type of gem has a max value for what type of souls they can fit.
I haven't started enchanting things yet, but good enchantments to use for weapons are cast on strike things like elemental damage plus weakness to that element, paralyzation, attribute draining enchantments, health draining, and weapon/armor eating enchantments. You can even make them dish out damage over time...
For armor its good to have constant effect things that fortify attributes, resistance to fire, spell reflection/absorbtion, chameleon or security, water breathing, swift swim, water walking, or levitation.
It's also usefull to have an item with mark and divine or alsmavi intervention to get out of tight places. And another with recall and a high invisibility enchantment.
Keep in mind that like spellmaking, you have to have the spell in your spellbook in order to enchant an item with a specific spell. Also when making powerfull enchantments you need high enchant skills and high intelligence in order to be sucessfull. If you fail, I think you lose the gem. To get around this, many people gulp several fort intel potions to bump intel up to 500-1000. You can also go to an enchanter and they will never fail.
Be carefull with enchantments, from what I've read people can make themselves so powerfull that they are godlike. For this reason I haven't started playing with them yet.
Here are two FAQ's for enchantments:
http://db.gamefaqs.com/console/xbox/...ent_points.txt
http://db.gamefaqs.com/computer/dosw...nchantment.txt