It seemes like the mobo you are looking to buy is Micro ATX. This means that it's smaller than the average sized mobo. Make sure that your components will fit, the Geforce FX 5900 is a rather large video card.
1. It depends on what version of XP you have bought. Some will only work with one particular computer (or computer part).
2. No need to buy a new HD unless you feel you want more space. In which case, you can always get one later on.
3. Yes. Especially when it comes to 3D graphics
