VSync: Vertical Synchronization (VSync) is the synchronization of your graphics card and monitor's abilities to redraw an image on the screen a number of times each second, measured in Hz - for full details, see this page. Whether LCD or CRT, your monitor will have a maximum rating in Hz for each resolution (e.g. 1280x1024 at 75Hz). When VSync is enabled (ticked), your maximum FPS will be capped at your monitor's maximum refresh rate at your chosen resolution, and more importantly in some areas your FPS may drop by as much as 50% if the graphics card has to wait to display a whole frame. Thus enabling VSync can have a negative performance impact. To offset this impact, you can enable Triple Buffering.
Unfortunately in Fallout 3 VSync is enabled at all times, even if this box is unticked. You can force VSync off by editing the iPresentInterval=1 variable in the Fallout.ini file to iPresentInterval=0 (See the Advanced Tweaking section). You should also check to make sure your graphics card's control panel doesn't have VSync forced on - whether via a specific Fallout 3 game profile or in the global options (see the Nvidia Forceware Tweak Guide or ATI Catalyst Tweak Guide for more details). Disabling VSync should be the optimal option, as it can increase framerates and more importantly, it will reduce mouse lag. However unfortunately in Fallout 3 it can also increase the micro-stuttering/juddering/skipping issue (as covered in the Troubleshooting Tips section). In any case experiment to see what your results are with this setting, though again, remember that it can only be turned off properly by manually editing the relevant line in the .ini file mentioned above. If you just want to reduce mouse lag without disabling VSync, see the Max Frames to Render Ahead tweak on the last page of this guide.
http://www.tweakguides.com/Fallout3_5.html
If you use my ini settings below your machine will scream!
http://pastebin.com/hwDdb5Au
