I would have to say it's all speculation right now because neither DirectX or Vista have been released. Though experience tells us that new software or driver sets really don't affect performance dramatically; unless the previous versions were really poorly written.
"Nagi, how do you feel about being 'normal'? Normal means that if you leave something alone, it just remains that way forever. So if you can't stand that, then you have to be "not normal" in some area... - Seiichi Kirima's dying words