A 4890 is DX10.1 not DX11. So DiRT 2 will only play in DX9 for you.
Not good I know, a DX10/DX11 version would have been a better solution for sure.
Then why does reviews I have read of the game suggest that it is DX11? Most of them mention the game will only run in DX9 or DX11 and point out the lack of a DX10 mode.