For Compatibility...

I always have the latest DirectX version on all my computers. I only have the DX 8.0 SDK on my dev machine.

It depends, though, on what market you're targetting. The game dev people will most likely have the latest version. I believe WinXP comes with DirectX 9 already loaded.. so you've got quite a base already. If you're targetting a usual gamer, it's more than likely that they already have the latest version from other games that have been installed. If it's a non-gamer who isn't really into computer stuff (such as the type of consumer realgames or valusoft target) and are still using win98, chances are that they're still using dx6.

Every single 3D game made in the last 3 years has required a card that supports DX7 minimum, and many require DX8 or DX9. This even applies to OGL games, as the features required to run a given feature in an OGL driver is going to be present on an equivilent class DX driver.

DX9 will work on Win 98, Me, 2k, and XP. That covers approximately 95% of Windows users, about 90% of the overall PC market, and 100% of anyone who actually uses their computer for gaming, because nobody that plays games on a PC will be running windows 95 or windows NT.

There is no point in worrying about supporting people who are still running win95 or winNT. the version of DX they have installed would be the LEAST of your problems when trying to get your game to run properly on their machine. Not even MS themselves support these OSes anymore; they're 10 years old.

For all other users, you can include the DX9 redistributable with your installer.

Oh, and:

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