Not too many people seemed to have played, or enjoyed it, it seems, but I am a really big fan of Empire Earth. It was created by the lead designer of Age of Empires. Has a very similar feel to it, but has a huge time span. Everything from the Stone Age, to the Industrial Age, to the Nano ago (2200AD). The graphics really aren't too bad, even for a game a few years old. Even with the 9800 Pro, a really heavy attack (especially a bunch of nuclear bombers) can still give the system a good run for its money with the graphics turned up.
Might wanna check out the demo to get a feel for it. I think they have one at
www.empireearth.com