The original Age of Empires plus expansions are still cool and definitely worth checking out if your into the second game.
Probably the biggest recommendation I can make is Age of Mythology plus the Titans expansion. Fun campaigns with a lot of variety, an easy to use map editor if you're into that sort of thing, four distinct armies and loads of crazy god powers and myth units. There's a lot to like here, however if your not keen on overspending I'd recommend seeing if you can find a copy in a bargain bin somewhere as the only version you can get on Steam is the remastered edition which costs more than it should and doesn't really add much worthwhile beyond Steam multiplayer.
You may want to check out Supreme Commander, a large scale mech RTS with loads of ground, air and naval units and Total Annihilation, the older game its more or less based off which you can get on GoG .com.
The original Company of Heroes and expansions are definitely worth picking up if you haven't tried them yet, they're more like Dawn of War 2 with more of a focus on the action and not a whole lot of base building but with all the expansions you get a lot of game for not a whole lot of cost.
World in Conflict with the Soviet Assault expansion is one of the most unique and fun RTS's I've played with great singleplayer and multiplayer modes. There are no bases, you can usually only control around a dozen units or so in a large battlefield and you need to split your attention between ordering your units around and using your own commander abilities, which include airstrikes, air-dropped tanks and infantry and even nukes.
Aside from that GoG is FULL of great older RTS games which run on just about anything. Total Biscuit has a big huge list of various awesome strategy games, including stuff like Sacrifice, a kind of third-person RTS where you play a wizard summoning and leading hordes of monsters, Ground Control 2, kind of an earlier World in Conflict style large-scale sci-fi RTS with no base building, big battles and a lot of varied units and Dungeon Keeper, where you act as a kind of evil overlord controlling minions and excavating and expanding your dungeon realm while fighting enemy heroes and adventurers.