WTH NFS : SHIFT RUN'S LIKE S**t ???

hi everyone recently I have bought a new PC :

core 2 quad Q9400 2.66 GHZ

4 GB ram

xfx ati 4770

I just install NFS : SHIFT and I can't play it on high WTH I played RE5 on DX10 with 51.5 FPS avrage and never go below 35 FPS but in SHIFT the fps go below 25 and it's look like **** ... grid on medium is look's better ... what's wrong guys ?!!