Should i buy a new graphic card for dx11?

At this point I would say no considering your current setup is still very good. DX11 is nice and some games use it but it's not yet at the point where it makes sense to upgrade just to have a few extra features in 5-6 games.
I would aim more towards upgrading sometime next year. By then DX11 games should be much more prevalent and sometime early next year there will likely be a new series of cards by ATI and perhaps a die shrink of the Fermi cards. Wait to see how that shakes out then make an upgrade.