With tessellation and such that DX11 features over DX10 and DX9, you'll see a HUGE difference. Tessellation will demand more out of your GPU and CPU. Not a lot of games have adopted DX11, and those that have are at least DX10 enabled games. If you're not ready for the DX11 jump, or want to save a little money going with a DX10 card, there is no problem with that. Here's a good link to show you the different between DX9, DX10, and DX11. If you can get a GPU powerful enough to use the features of DX11, I would highly suggest it.