hi Mattley, excellent choice, i also have gone digital because i've no more room for books. your wife is lucky you're giving her such a thoughtful gift, especially at a time when she'll be able to get a lot out of it !
i'm sure you'll get plenty of advice about which one to buy but i recommend you list your location (just a country or region is fine if you don't want to be too specific) because some choices depend on that (for instance, the kindle is only sold in the US for the moment, and the wireless download feature will not work outside the US).
welcome to the forum ! take a look around the "which one should i buy" forum and also check out the "e-ink comparison matrix" in the wiki (see the link at the top left of the page) to get some infos on the different models available.
and of course, once you've got one, don't miss our ebooks section where you can get thousands of free public domain books, all formatted by our members.