AGE 2 was on sale?! missed that, it was my favorite age of empires(though never played 3)
I don't think it's on steam, unfortunately. Just saw AoE III and was like... eh...
Strategy and action screams Warband. With confetti. And a slightly intoxicated man in a chicken costume holding a burning sign. Also balloons.
Check out Natural Selection II for an excellent sci-fi team based FPS/RTS... unfortunately I think the sale on it is over, but it'll go on sale again eventually.
Torchlight II is a good game, if you like Diablo style games without all the Diablo III shit along with it. Again, missed the sale. :/
Metro 2033 is a good single player FPS, if you can get it on sale. It goes for 75% off frequently. I guess it's kind of sci-fi.
May want to give Endless Space a look, if you're interested in 4x games. I've heard mixed reviews, and can't say I've played this one personally, unlike most of these. Blurg, no 4x.
The Elder Scrolls games would hardly be amiss. Try whichever of the older ones you feel you'd enjoy more before buying Skyrim. Even if you do buy Skyrim, the older ones are still good for their age, and if you don't like Oblivion or Morrowind, it was only five bucks lost. Again, didn't play these myself, but they're pretty much unanimously loved.
Terraria's alright. There's a good deal of single player content, if you're into the kind of genre. If you've played minecraft, think of it as that, but with an actual game with, you know, stuff to do.[/cheeky]
Dungeon Defenders is only really fun played with friends, in my experience. I've almost exclusively played in private games, so if you don't know anyone who has it and would still play, I'd give it a miss. It also has a metric fuckton of DLC.
Orcs Must Die is arguably better than Dungeon Defenders in some respects. I'd say it's a better single player game, while still being quite fun in co-op (for the second one, the first doesn't have multiplayer, I think). I liked the progression more than Dungeon Defenders' loot system, anyway.
Sim City 4 is pretty great if you like city building games. If you like Dwarf Fortress, which I'm guessing you do, then you'll probably like it. Missed the sale on it though. But, since there's a new one on the way, it will most certainly go on sale again soon.
I'd recommend Dishonored, but not if this money is meant to last. Wait for it to go on sale after a while, if you're into actiony kinda stealthy beard stabbing simulators.
Chivalry's quite fun. Its combat system is kind of simple, but well done. War of the Roses is more Mount and Bladish in many respects, and since we all know you've bought warband by now (HINT HINT) you can decide whether either is worth it.
Oh god, what else... uh... Portal, but who hasn't played Portal? Also the new Counter Strike game. It's pretty fun, and lives up to the name better than CS:Source did, imo. It's also pretty cheap, but again, wait for sales on all of this. There are some good free to plays on steam as well, but obviously you can just try those for yourself anyway. The only ones I've had any experience with are Tribes, Deadlight, and some Planetside 2.