A GT 240 isn't DX11 capable, you'll need Fermi for that (GeForce 400 or 500 series) or Radeon 5000/6000 series--not to mention the 5670 isn't powerful enough to play games with DX11 turned on at any resolution larger than 1280x720 or so.
If you give us your budget, what parts you own, and what you're trying to do, we can try to give you some suggestions for your best options.