DX9 is still supported because the PS3 and X360 only support DX9+ level features(PS3 doesn't support D3D at all btw), it has nothing to do with older cards. DX 10 and 11 ship from Vista not what you said and since DX11 support feature levels there is no reason other then the consoles to support a DX9 path. We dropped PC DX9 support with DiRT: Showdown, but where thinking of doing this on DiRT 3 already.
Don't take my word from it, but i've heard that Directx 10 is kinda, well... not used much. Directx 9 or 11 are the most used ones. Why you choose Directx 10?
I believe that DX9 is supported still simply because older cards still cling to it but aren't up to DX10, DX10 is for newer cards on XP, DX11 ships with Win7 only.
DX10's advantage over DX9 is more shader flexibility. And a little performance. But that's about it. I really cannot claim to know more on the subject difference between the two.
Then start coding for 11, nowadays learning DX10 has no uses what so ever, jobs ask for DX11 or OpenGL 4 now. Also DX 11 fixed a few things that DX 10 got wrong.
pls anybody teach me abt placing this so called 'virtual camera' and also about its projection matrix in detail
i choose direct x 10 because i have bought the book for it .... I have also found that 11 and 10 are almost same (from program code point of view) and i thought it would be easy for me to get used with it.