Unless they received THOUSANDS of dollars in iTunes gift cards they didn't get more than they can possibly use!!!!! Even then, I could EASILY buy "upgrades" and "tokens" for games to spend fun money (if I had it) :-)
If they have the better iPods they can buy apps.... what type of ipod and what size is it?
Even the SMALLEST ipod (the mini) is 2gb which is about 500 songs.
Unless they just aren't "into" music (which again.... I don't understand. I don't know ANY kids that aren't into music), or are quite young (like 5 or 6)... then you might have a problem.
They can buy books, too. You just buy the app and then purchase the books.
You can load the gift cards into iTunes so that you see the balance getting smaller as they purchase. If they don't each have their own iTunes account, you'll want to keep a spreadsheet of purchases so you know who spent what.
The other thing that you can do is regift them. YOU "buy" the gift cards from your boys and then give them as gifts throughout the year. ANY Tween on up would LOVE an iTunes gift card.
I wouldn't swap - you typically lose money on those sites. I'm not sure how that works, though.
If you go to www.apple.com/ipodclassic/specs.html it will tell you for each iPod what the capability is (for example can you buy books, movies, apps etc). That would at least enable you to determine WHAT your boys can buy.
Have fun!