One optional note for the Robe or Mantle (cloak can't get 4 sockets): I'd suggest farming Normal Cows for rare charms, use those to cube the Robe or Mantle until you get good mods on a Rare Robe/Mantle so that after you put in the Humility or Chains of Honor rune word you can then temper the armor (cube with 3 hearts) for 15%-20% life leach.
Cubing with potions gets you a Magic not a Rare, and Magic can't be tempered. On the other hand, potions are more readily available than rare charms. So I'd do as Shilos suggests until you can do as I've suggested.
Amulet: socket whichever you have with a Zod to boost resists 14%, magic resist 7%, and increase maximum resists 7%.
I agree with the Diva set recommendation, be aware that the items have variable bonus (+1-2 all skills), so keep gambling until you have +7 all skills from the whole set.
Cube everything you have that has a "defense" rating with a Ghost doll, then with an oil of permanence.
What merc to you have, and what is his/her equipment? I'd recommend an actIII cold sorc so the cold slows most opponents, and equip with things like Shadowtrick and Baals Claw for conviction and holy freeze auras. Many opponents will lose the "Immune to Physical" attribute when this merc hits with an Ice Blast. Equip this merc with a Diva set also, and you'll then be able to progress in Hell.
Note: the dungeon under Nihlathak(sp?) has 5 "Nasty Place" areas, in these areas you can find Vile Witch monsters that can drop soul stones. These are very desireable for resists and +allskills. Also, the "Red Portal" areas in act 5 (the portals are blue in CE Mod) are home to clones of Diablo, farm him for an Annihlus(sp) charm, the have a +2-+5 allskills bonus plus resists.
You may find it wise to farm NM Cows for grand charms to re-roll for +skills.