The biggest bottle neck is not your video card but your CPU and RAM.
I had a Ti4600 with a P4 2.53 and 1 gig of Rambus 1066 ram. And I still seen some slowdowns. not that much but more then you would expect. This game uses an engine that just does not cut outdoors high polygon maps.
I have since upgraded to the so called Nvidia killer ( lol ya right) a ATI 9700 pro. I only gained 10 -15 fps. This card was to kill the GF card in every test. But that's another long story.
Make sure your system is running at optimal performance. Everyone gets some system lag now and then. You just don't get it as bad as other people with older and slower cards.
You can try:
defragging your hard drive
get yourself a copy of adaware and remove all unwanted spyware on your system
make sure you dont have 30 icons in your systray running 30 programs in the background.
use a refresh rate program. even thought you think your game is running at 1024x786 @120Hz it;s only running at 60 when playing a game. A refresh program will make your game run at your desktop rate.
when you installed your latest OS did you do a upgrade or a fresh install. a fresh install is always better.
the link to beta drivers and the refresh fix is here:
http://www.guru3d.com/files/