I know it's not an RTS. but if your into strategy and complexity of gameplay. Give the Civilization Series a go. It is turnbased, but terribly addictive and fun.
You can pick up CiV III on Steam for £2.99, which is my personal favourite. CiV 4 is on steam for £12.99, or £24.99 with the expansions. Both fantastic games IMO.
I liked playing Company of Heroes online. WWII themed, a lot of the game isn't just building/sending troops to attack *like in Sup-Com*.
Infantry units need to be placed stratigally to win (your units take less damage if they are by a wall for cover, or your units do more damage if you send some to attack at the flank or rear of your enemys.)
Armor units have weak spots, so moving tanks to attack is an art, not just point click and forget. You gotta be able to adapt to different units and scenarios quickly to survive.
Much better RTS engine than Star-craft / Supreme command simplistic junk with just mass troops smashing into each other and knowing what/when to build the fastest to win.
Company of Heroes also has good DX10 graphics, i'd give it a try if I were you. Online play is pretty fun. Single player on max AI can give veteran RTS players a hard time until you learn how to play.
Oh, and different commander options is fun. As americans you pick to specialise as Infantry, Armor, or Airborne Paratrooper specialties. Lots of fun, and few different ways to play to fit your style.
I'm not HUGE into RTS, but Company of Heroes I found to be very fun. *I hate Starcraft-type RTS......*
Let me refrase that, I dislike all the garbage that Blizzard puts out.
Think the last game they put out that was worth anything was Diablo 1, and even then it wasn't that impressive imo.
Company of Heroes is a great RTS. Sins of a Solar Empire is still my all time favorite though. [Though a bit once sided late game, when those mass drivers start hitting you planets from the other side of the galaxy...)