I would simply get Windows 7. You can then force the game to run in Windows XP SP2 or SP3 mode. This is done by right clicking the game EXE file, then click Properties, go to the Compatibility tab and from there choose Windows XP. This will ensure DX10 and DX11 is disabled for the game.
Of course if you want to play a game that does not support DX9, like Just Cause 2, then doing this will cause the game not to run.
but on windows 7 it uses much more resources than windows xp but the directx runs the game so which windows would be better ?