Really, processors aren't getting that much faster -- the bottlenecks are becoming the hard drive (and associated -- e.g., sata controller, etc.) and the memory. They're evolving both sideways (I.E. quad cores and hex cores), and with the whole rearchitecturing thing (i3, i5, i7, etc. vs. previous pentium-based architectures).
In other words -- any of those should be fine, even the i3. For things like email, web, desktop production (especially what I would call "consumer level" desktop production -- graphic artists are a whole different thing), your problem won't be the machine, it'll be other things (I add "network" to my above list of examples -- especially for web-browsing).
The only time you need a more powerful machine if you're doing something moar hardcoar -- gaming, number crunching, video processing, image processing (which would mostly be memory), music processing...and someone like me, who really doesn't do anything, but just craves power. Don't feel obliged to do what I do.
If you do a lot of multi-tasking, then I would concentrate on boosting memory (including making sure she gets a 64-bit operating system), and getting moar coars.
Sorry, mah cat is helping me tahp.