While it's a bit different than most RTS games I highly recommend Hegemony Gold: Wars of Ancient Greece. This game is less about base building (in fact you don't really build bases, you just take them over and fortify them) and more about epic conquest. It's not a bunch of little levels like most RTS games. It's one HUGE map.
While it seems quite simple it can be quite complex. I think it's a great deal because it takes a LONG time to beat the game and even longer if you want to take over the whole map. I think it took me ~150 hours to complete the main campaign and take over the whole map.
The mechanics are like nothing else I've played (though I've heard there may be some other similar games) but they are not hard to learn. They finally got this game on Steam recently so you may be able to pick it up cheap if you wait for the summer sale.
Also it should run on your system. The only thing I'm not sure about is the video card. This game is not very hard to run. I know someone who played it on a Sempron 64 with 1GB RAM, and a Radeon 4870 and it was slow but playable. You have a much stronger CPU and more memory but weaker video. I would still think it should run ok.
Unfortunately I don't know of any other recent RTS type games that are any good. That genre seems to have lost popularity which is a pity because I love RTS games. Just stay away from Age of Empires: Online. I found it horrible compared to the previous AOE games.