I think you'd be fine with either OS, although you might have an easier time with XP. I do recall that many multimedia applications like having .NET under Windows, and Vista has an advantage in that department because it is built-in (and thus one less thing to worry about). Most of the top tools should work fine on Vista, but I would look for Vista drivers for your related devices before making a decision. I should also mention that Vista has some movie/DVD-editing software built-in that is quite nice, plus Vista x64 is quite nice with a powerful system with a lot of RAM if you're planning on serious editing.
The programs I've tested under Vista x86/x64 are Adobe Premiere, Ulead VideoStudio, and Pinnacle Studio 11, just for your reference.