Starcraft 2 is probably THE game in the RTS genre. Its Mac Version is also quite good and not some cheap port. It is the only game I really enjoy playing on OSX.
Generals is also really old. If there isn't a Mac Version, that game might just run fine in a virtual machine. Getting the resolution right is probably the biggest issue you will come across. In Multiplayer (LAN) Generals is really picky so a Mac Version might just not work together with any Windows versions.
SC2 is a good still fairly modern game with the Starter Edition quite cheap. And that is really all you need if you don't want the single player campaign. I think they also offer LAN spawns, so you can play with friends at a lan without them actually owning the full game with full features.