You can only improve performance (that is, the number of frames per second that your video card spits out) by adjusting the level of graphics detail (or buy a new card, but you've already done that!). The ingame menus do not require much graphics power, hence the apparently high framerate. Tone down the detail and any AA/AF settings in the control panel.
You cannot "adjust" FPS directly in most cases - the performance you see is governed by the processing power of the video card and the settings you apply in games.