You'd have to experiment, but I have roughly the same system as you and I went nuts and created a several hundred star "galaxy" with 10 players and it ran fine. Note I said star, each star had from 2 to 60 "planets". However dunno if it would be feasable to play it when the races actually started meeting each other. I had them scattered all over so the likelyhood of such happenening wouldn't be for a very LONG time. My computer could handle it fine (though it did get this weird lockup/pause once after about 3-4 hours play time, where I had to tab out of the game (took a long time) do something else on the system and then back in.) I did this mostly to save at that point, but it played fine for another few hours. What I mean is advanced game you'd be hit on so many fronts (and doing your own attacks) that I think it would become extremely chaotic to play for YOU, the computer I think can handle it.
Here is an example of what I mean I did a smaller scale. I played I think it was Gateway (scenario), has only 6 players and I control roughly half of the first galaxy (err star) and there are only 2 computer opponets left now, one has roughly half as well and the third is barely hanging on (controls only 2 systems). No one owns any systems in the second galaxy (err star) yet as I've been blocking the gateways system. The big problem I'm having with that game atm is between my "expansion" and the computers expansion there are SO many systems needing attention that it is becoming problematic to manage them all. So what tends to happens is I excel where I'm Focused and lose ground where I'm not, then I stablize my focus area and go back and make up where I lost ground and so on. Late game it's becoming very tedious. lol
Anyway, my long winded post is, find out, but I think you'll find you can handle much more then you can manage, your system will, you on the other hand....
Oh one thing, I have a nVidia 8600GTS (256mb) and I play with everything set to maximum at 1600x1200 with an FSAA of 2, runs fine.
