DX10 = DirectX 10 its the latest version of making pretty graphics in laymans terms... Exclusive to Windows Vista only at the moment - only supported by latest graphics cards Nvidia 8xxx series and ATI R2xxx series.
DX 9 = The current widely used version of DirectX...
http://www.winmatrix.com/forums/inde...showtopic=9550
For a quick visual on whats the biggie just look at the screenshots... DX 10 supposedly allows for not only better graphics but more efficient use of GPUs because they are equipped with Stream processors that can adapt to Pixel or Vertex shading on the fly as opposed to Current DX9 hardware which has a set amount of Pixel shaders and a set amount of Vertex shaders.
In laymans terms - it should offer an overall fast and better gaming experience...
BUT Current DirectX 10 titles are very slow performing compared to their DX9 counterparts so you need a higher range card like the 8800GTS...
I recommend getting the 8800GTS if only to play the most anticipated PC game of the year when it comes out - Crysis... its a big title and your pc will have the power to play it with the ram/cpu you just need the graphics..