[I am aware that this is an old thread, but I am going to answer it anyways for those who are also wondering the same thing.]
The two items most valuable when upped, as far as I know, are Arreats Face [Barbarian Helm] and Herald of Zakurum [Paladin Shield].
When Ethereal, Zod'd AND upped, these two items are virtually priceless on non-ladder. Let alone if their ED is perfect.
I have also heard rumors of older versions that were Zbugged back in the day to save their open socket [.08?, right?]. Zbug was an old exploit that allowed players to remove a Zod from an item and still retain it's "Indestructible" mod, therefor saving it's open socket for something better.
An Eth, Perfect ED, Upped, Zbugged HoZ or Arreats is pretty much a myth nowdays. Although they exist, they're prized by their owners who are but a few and you'll probably never feast your eyes on one.
Socketed items in general are good to up if they're "exceptional" rather than "elite". For instance, say you have a 4os Ornate Plate with 15% Ed. That could be better upped to a Sacred Armor. However, if you plan on using it, and your merc isn't above level 89, it's better off staying an Ornate or even just using a different armor in general such as a Dusk Shroud or Archon Plate. This is because a Sacred Armor requires over 200 strength and an ornate lacks defense when compared to DS and Archon.