Airport Extreme better than Express - so unless you're on a really low budget go for former. Express is good, I use both, but should be thought of more as adjunct to iTunes. Extreme has more external interfaces and more features so it's better suited to digital hub with printer. 802.11g is fast so won't slow down your broadband.
Suggest you buy your wife a Mac mini with Airport Extreme card - that way you'll be sure no compatibility issues. It's Valentines day after all
