I have recently formatted a new system running Win7 and a HD6870. With the install, I have direct X 11 and I was wondering if I will face any problems after having installed the Crysis demo which installs DirX10.
DxDiag shows me running DirX11 but I was wondering where the installed files from the Crysis Demo went (if there are any floating around affecting performance.) When installing the demo it said installing Direct X which I believe would have to be DirX 10.
You are allowed to have multiple versions of DX on your computer, as not all games use the same version. Crysis installed DX10 because that's what it uses. Other games, such as Resident Evil 5, you are given the option between DX9 and DX10. Or, newer games of which can run DX11. Each version offers difference amounts of quality in graphics as well as performance.
To answer your question, you shouldn't experience any performance issues from having multiple versions of DX, as only one version can be used for any given game at any given time.