You are limited based on what bonuses you wanted. In my case I really wanted the material/opposition/endor which fit in best with my build. I am using these items:
Necklace: Material/Opposition/Fire: Fire Resist 50 (enh), Fire Resist 25 (insight), Fire Resist 17 (competence)
Belt: Material/Opposition/Negative: Blindness Immunity, Save against disease +4 (insight), Fear Immunity, Save against Poison +4 (Insight), Deathblock, Negative Resistance 25 (enhancement)
Gloves: Material/Opposition/Earth: Acid Resist 50 (enh), Acid Resist 25 (insight), Acid Resist 17 (competence)
Boots: Material/Opposition/Water: Cold Resist 50 (enh), Cold Resist 25 (insight), Cold Resist 17 (competence)
Bracers: Material/Opposition/Positive: Unconscious Range (128 enhancement), Positive Healing per 10s (16 enhancement), Unconscious Range (64 Insight), Positive Healing per 10s (8 insight), Unconscious Range (32 Quality), Positive Healing per 10s (4 Quality)
With the Material/Escalation, Ethereal Opposition, Ethereal Escalation there are only 3 different types of items so getting the 5 piece bonus requires redundancy.
All the possible bonuses are listed here along with the bonuses:
http://ddowiki.com/page/Legendary_Green_Steel_items
Ethereal Dominion is another really good caster choice as it gives a big spell point bonus and ghostly bonus along with all the element, positive, negative crits. With my build alignment/light and force were main damage types so that set wasn't as good for me.
But then I think the fully stacking damage to all spell types is actually a little better anyhow.