Soleron on 18 March 2010
There is not much difference, with games today and for the foreseeable future. DX10 and 11 bring small eye candy upgrades to those who have the hardware and Vista or above, but nothing worth upgrading to unless you're already buying the new card for its processing power.
In the future, DX11 (and OpenCL) may be important for GPU compute, but that's a niche market right now.
Basically - are you happy with your current graphics performance? If so, stick with your current setup.
