It will be focused more towards Vista, if they're planning to include the 360 and the PC gamers across the same servers and not split them into two separate groups then Vista is a must. After all that’s what Microsoft is supporting for cross system gaming. I'm no way saying XP won't work or be supported but I would expect them to support the newer features that are available. There is no point in making it for an older system spec that will go out of date eventually, the game market always future proofs themselves in some way or another.
Also considering it will be using the UT3 engine the graphics cards you’ll need to play at minimum will need to be able to use shader model 3. So if you can't run games like Splinter Cell: Double Agent or Rainbow Six: Las Vegas its a good bet you'll have issues with the game when it's released.
I prefer XP but I'll swithc over once I know it will be worth my time and my money. This is of course my own opinion and conclusions, so it could be miles off and they could support Linux. :P
J/K we'll just have to wait and see.
