Hello Mahmoodifar & Welcome to Sevenforums.
First of all, try updating your DirectX! Most games are solved just by updating your DirectX.
Check this topic out for a lot of solutions that could slow, drain etc your game.
Tips on Troubleshooting Game Issues
I'd also like you to redirect to this page.
There's a thread same as yours. Same game also. Check it out (EDIT; Sorry, no answeres there)
64 MB GPU Memory Only ?
It could also be that the rest of the memory is 'borrowed' by your RAM.