As with any new OS, Vista will,one day, become the OS advisable to have. (Well, as long as nothing goes wrong with it after release) Just Like 95,98,2000,ME, and XP before it. As times change, so will our OS's, but i wouldnt start worrying just yet.
Even when Vista is released early next year, XP will still be a HIGHLY used OS, and will be supported with critical updates for a good couple of years after. So dont feel you have to rush out to by Vista, you will not see XP vanishing anytime soon.
And im sure i am right in saying, all of your XP questions will continue to be answered here at BC long after Vista is released.