Sorry, can't help you with headers as i haven't used directx. But as for their version, you should still use DirectX 9 headers. They work fine with every graphics card, even old ones. DirectX9 just brings new additions that you can use or make them available to others who use your programs, but basic functions should work for everyone.

Try not to make just another DelphiX It use DX7 and everyone's complaining about it.