Sci-Fi is what I play.
http://www.unificationwars.com/
Try this. It's a little confusing, and although you don't do anything when you are out of turns, its pretty fun. I suggest this, its a good RTS. The stuff you pay for (just unlockables) is so little used because it often isn't needed. In Unification Wars, you play as an emperor trying to conquer the galaxy with your ships, planets etc.. I'm very sure it's an MMO as well.
rts=Real Time Strategy not turn based...i have played this, it was ok but i'm more into real time then turn based, i guess i could try it again...
also Shell, terraria requires atleast 128 mb on the card and pixel shader 2.0? and vertex shader 2.0 or it's 1.0 idk
edit: anyone ever play Total Annihilation : Spring? it looks good...