To run DX9 games you just need to install the DirectX 9 runtime and you should be good to go. I have ran a lot of very old games including some that needed DX5 with absolutely no problems at all with Windows 7.
"You will never have more than 1 version of DX in Windows OS."
Not completely true if a game requires DX9c and you have not installed it the game will not play. DirectX 9 and DirectX 11 can co exist in Windows 7 just fine. It is amazing after all this time that Vista and Windows 7 has been out that people still think they can't install DirectX 9 just because the OS came with a higher version. You can even install DirectX 9 in Windows 8
i have just got bioshock infinite and when i clicked on the icon it says "Bioshock Infinite requires that your graphics hardware be compatible with directX 10 or directX 11, and have the directx 11 runtime installed." i have downloaded dxwebsetup.exe but dont know what to do. also i have a gforce gt630m on my laptop if that helps