I never played the demo, so I'm not sure what new stuff you'll get.
I have to say SupCom is the only RTS to really suck me in since Starcraft - that's extremely high praise
. Then again I've never been much of an RTS junkie.
The game does so many things that every game should do... The waypoints are so intuitive, the ferry system for transports, repeat build orders for factories, and the ability to scroll in and out is so incredible.
I have a better video card than you, but a worse processor. I think you should be fine running the game at whatever setting you desire. I see no difference in performance running at highest vs lowest settings. It seems to be a very CPU-intensive game, and lowering the graphics doesn't seem to reduce the lag I get at all.
I can play a 1v1 game and not get any lag till both of us have 1000 units a piece. I have a feeling I might start to get some pretty bad lag in a 2v2 or with more people, but I haven't ever tried that.
You ought to be able to run it fine.
I highly recommend picking it up - sounds like it's exactly what you're looking for in that it's a new and creative RTS. C&C3 is (as far as I can tell) just about identical to all the other C&C games so I don't think you'd be very happy with it.