First, look at the minimum requirements for what it will take to run. They're not much different outside of needing a video card with Pixel Shader 3.0 support. 
If you can run Diplomacy and have that, you'll be able to run Rebellion - albeit perhaps at lowering quality settings. The bump in processor speed is simply because there's more going on, but again, if you can run Diplomacy right now, you'll be able to run Rebellion with what you've got.
The game should run fine under Windows XP SP3, but we are currently not planning to officially support it.
The requirements may change once we get further along in development and see how much further we can optimize things.
