ok in nfs shift on my gtx 260 when i activate 8x anti aliasing in the game menu and application controlled anti aliasing in control panel the fps sometimes drops below 40 in the replays.but when i activate 8x anti aliasing in control panel by enhancing the application setting and setting 4x anti aliasing in the game menu the game gets a lot faster.so is the 4x plus 8x control panel less effective in jaggies than only game menu 8x aliasing?
I believe by setting the anti aliasing to 8x on the control panel and turning off application anti-aliasing, even if in the application you still set it to 4x, it will still be running on the Nvidia control panel settings, which is 8x (I could totally be wrong here, as I'm at work and can't confirm this, but that's what logic dictates).
Easy way to test this: Set the nvidia control panel to 8x anti aliasing, and then turn off anti-aliasing in NFS Shift and see how it looks. If you see no difference in visuals from when you have it on Application Settings in the Nvidia control panel and NFS Shift's in game setting of 8x compared to the opposite (8x AA on the Nvidia control panel and 0 AA in NFS's settings), then that means that for some reason your Nvidia control panel's rendition of 8x AA is doing a better job than the software rendition of it, for whatever reason.
If you DO notice a huge graphics difference between 8x AA set in game and 8x AA set on the control panel, then the application is rendering it's settings and overriding whatever your control panel is set to, which would explain why your 4x AA setting in game, even when your control panel is set to run at 8x, runs smoother. Some people may have difficulty telling the difference between graphics running on 8x AA and 4x