If you take your insights from O. Henry, then you will conclude that Jim and Della were incredibly wise. Here are the words of O. Henry at the conclusion of the short story:
"The magi, as you know, were wise men--wonderfully wise men--who brought gifts to the Babe in the manger. They invented the art of giving Christmas presents. Being wise, their gifts were no doubt wise ones, possibly bearing the privilege of exchange in case of duplication. And here I have lamely related to you the uneventful chronicle of two foolish children in a flat who most unwisely sacrificed for each other the greatest treasures of their house. But in a last word to the wise of these days let it be said that of all who give gifts these two were the wisest. O all who give and receive gifts, such as they are wisest. Everywhere they are wisest. They are the magi."
If you examine this short story from this perspective, then you can say that Della's heart to sell what was most precious to her, namely her hair, to buy Jim a chain for his watch was wise. Similarly, when Jim sold his watch to buy a set of combs for her hair, he, too was wise, because his actions were rooted in sacrifice as well.
What makes all of this giving even more beautiful is that Jim and Della are portrayed as a poor couple.