DirectX10 question

I have a question. Okay, Microsoft seems to be talking about DirectX 10 as replacing DirectX 9. Will DirectX10 require a new graphics card? If it does then it annoys me greatly that nVidia is selling $500 cards and encouraging people to buy two of them and use nFocr4 SLI technology on them. They say “Be prepared for the future!” But if the cards aren’t supported by DirectX 10 then your $1000 worth of video cards is already outdated when Longhorn comes out… Seems dumb to me, no? It makes me angry when people who didn’t do as much research as they should’ve waste money.