Computer prices are much cheaper for alot more power than you could get a few years ago (I suppose that's always true). Apple is very overpriced to upgrade the options on their website store.
- Your mom won't notice a difference in CPU power, so get the 2.4GHz model.
- Get minimal ram and swap in your own (unscrew the bottom and it's right there).
- You can also swap in an SSD for your mom (lots of folks have). Helpful guide here and if you pick up your SSD here, you can get all the tools you need to install it (click on the size you need and you'll see the tools pictured).
If your mom likes her new monitor, the Mac mini is the way to go. You can run OSX or Windows or both. Also, if your mom does not like it, you get 30 days to return it for a full refund (minus shipping).