Obviously, this belongs in the Newbie section.
The hardware acceleration and Dxdiag FPS override options will NOT slow down the game, just the speed that the cideo frames are updated.
Aka, now you get 100fps, and after screwing with your settings you will get 50fps, but more action happen between your frames, you just won't see it
Any decent games made in the last 8-10 years (and Dx8 is only about 3 years old) have been coded to ensure that the game speed runs the same on all supported hardware (aka timing is the same).
A higher FPS is a good thing, and is independant to the game speed, it only tells you how many frame per second your video card is showing you. Anything about 60FPS is fine, anyting less than 40 is practically unplayable.
The game speed setting wil be in the game options, assuming the option is available. Otherwise you will just have to work on your reflexes.