OP has vista, therefor DX11.
I did the math and she will see a 22% increase in frame rate. The formula I used is based off passmark numbers, FPS % gain switching from DX9 to DX11 and the maximum gain achievable using a gtx580@1920x1080, which is 34%. The gain is noticeable and the only reason I upgraded from XP.
http://www.videocardbenchmark.net/high_end_gpus.html
http://www.tomshardware.com/reviews/...ce,2793-5.html
I have explained