Dual screen
We've already mentioned that Supreme Commander has a dual-screen capability, making it one of, if not the first game that you can pwn from two monitors at once.
We spent the majority of our time playing the game with two monitors. However, what we also need to mention is that you can employ a split-screen setting to get a similar effect on just one monitor. Of course, how effective that will be will depend on your monitor.
We found that split-screen was a great way to play on both 30" displays and 24" displays, with a resolution of 1920x1200 or higher. Any physical size below that and you're going to be better off playing single screen, since the individual 'virtual screens' will be too small to be of any value.
One thing we disliked about dual-screen, both physical and virtual, is that one becomes your 'main' screen and the other your secondary. Your main screen has the user interface, and you can zoom, scroll as normal. Your secondary screen doesn't have an interface, and doesn't allow you to scroll around the battlefield using the arrow keys, just zoom in and out on different bits. This makes it more of a 'secondary' rather than 'second' screen - you end up using it more to keep an eye on things than to actively work on it.
Given the scale of SupCom, and the fact war can be happening in multiple locations at multiple times, we'd love to see this second screen just become a functional replica of the first, allowing you to truly work on two parts of the battlefield at the same time, rather than be constrained by what feel like artificial limitations.
However, this is really just a gripe, and doesn't take away from the fact that dual-screen RTS is an absolutely awesome experience. Here's some pics of our conquests.
Our leet 8800/X6800/Dual 24" setup rocks Supreme Commander.You also need to bear in mind that double the screen real estate means double the rendering horsepower - you're going to need a reasonably high-end card, like a 7900 GT, to drive two 1280x1024 displays, and anything above that is going to require an 8800 GTS / X1950 XTX to get respectable frame rates when the lasers start flying. If you're running something like an X800 or 6800 or below, you're going to be stuck with one screen.