Starcraft II.. only good RTS to come out lately..
As for the learning curve, it's easy to learn, and hard to master.
Matchmaking will make sure you are in the right category (bronze, gold, platinum, etc...) so you're not put against harder opponents; when you get better, you will be moved up in ranking.
And when you get bored of the regular game, there's the dozens of custom maps (more like mods) for you to play. They range from RTS mods, to RPGs to mind games like Mafia http://en.wikipedia.org/wiki/Mafia_%28party_game%29
I think OP's computer should be able to run it fine. My old PC could run it decently enough (it was playable), and it was a piece of crap.
Maybe Civ 5, though it's not really Real Time. And now that I think about Shogun is more of a hybrid.