I'd just stick to XP as it's tons cheaper and you can probably get a free copy, plenty of them flying around and DX9 still has plenty of life in it, and as we've seen in the Crysis config hack, you can get "DX10-exclusive" features to run under DX9.
Get XP now and then get an upgrade copy of Vista later on which will be cheaper because it's an upgrade copy and by then prices would probably have gone down a little. Also, it's not really fully optimized right now and I don't see it as a necessity at the moment and not for a long while. Any developers who make a game Vista-exclusive are essentially shooting themselves in the foot.