Supreme Commander is definitely the way to go. You should also try expanding the game to two monitors which can be activated in the visual options. Believe me, that's going to be very very ... stressful. I did exactly that to find the limits of my new bought machine a year ago and it worked great, 5-10 frames maximum in a big map with ~5000 units already.
One stupid thing about that game is and I don't exactly know why that happens, if you got too much units on the map the game will slow down more and more over the time, at some points you won't be able to play it any longer since the movement of a single unit can take minutes for a path of 20 meters. Imagine 2000 units moving from one end to the other which took me often 30 minutes or more.

It's not fun as soon as you're running the game with speed +10 (the maximum afaik) and you still cant play it "fast".

Anyway, 2 monitor support and a big map, this will do the job.