With that much ram, 10.8 will run fine on it. Apple has released 10.8.2 ( essentially a second service pack now) so many of the shipping quibbles people had have been addressed. Ignore Bruno's whining ;), for what your wife needs, it will work fine. Been running it since release with no major issues.

Easiest way to go about it is to get 10.8 first, (Apple Menu, Mac App Store) let it run the upgrade, then after the upgrade run the Migration Assistant (click the magnifying glass in the upper right to search, type in Migr... Until it comes up, then click to run). It's is a painless way to copy over any of her data from either Mac, right over the network. Simply run it, read the instructions to pick what to migrate and let it run for a little bit.

*edit*. Migration Assistant also works to pull data from Time Machine backups she may have from the older macs. Pick whichever route is easier, network, or using he backup drive.