I still think it's just a marketing ploy for M$, forcing gamers to move to vista for halo2. Given how halo2 ran on the exact same hardware as halo2, it would be logical to think that a system which can run haloPC with good framerates under DX9 would also be able to run halo2 decently under DX9.. but of coarse.. all M$ seems to care about is, well.. thats it, the $.
I hope someone makes a crack for halo2 to run on XP, just to smite microsoft.