The DX factor is all controlled by the video card (and partially OS). When your ready for a native DX10 card, buy one. They won't be released until Q3 at the earliest, Q4 is looking more realistic.
Exactly what me and some otehrs have been saying. If they released DX10 w/ the initial release of Vista then the whole DX9d thing will be washed out and kinda pointless.