If you like RTS you should hunt around for Empire Earth. It's a really old game but GOG.com had it for free a few weeks back. We've been playing it recently and it is a pretty good game.
For $6 it's like getting Age of Empires and C&C Red Alert at the same time.
The only thing is that you need to have a bit of time to play it. If we play 2 v 1 or 3 v 1 against a medium computer player, the round usually takes 2 - 3 hours. The AI is pretty resilient in this game so if you don't completely wipe them out they rebuild very quickly and will start attacking again.