i've found that it really doesn't matter what version of directX you use with whatever video card. it shouldn't hinder performance. the only downside i believe is that your older card won't be able to reap the benefits that newer DX9 compatible cards can by using DX9. in short, simply installing DX9 shouldn't change performance, but if you're using DX9 games and applications, performance w/ that particular card will be less than par.
for instance, i'm using DX9.0b as well, and my card is a gf4 ti4200, which is only a DX8 compatible card. i witness no performance hits in any games or apps i currently use b/c they're all based on DX7 or DX8, and no DX9 titles have hit the shelves yet. but i know when they do, and i try to play them on my computer, performance is going to suck using this ti4200 regardless of whether i have DX9 installed or not, b/c my card is incapable of reaping the benefits of the features DX9 implements. needless to say, i'll be upgrading to a DX9 card soon...