fps limit in OpenGL?

Yes it is, it is called the V-Sync. It limits the frame rates but it's due to rendering the frame without any glitch or tear. What it means that in order for the graphics to produce a good visual during 3D applications the V-Sync does it well obiously by syncronizing the vertical and horizontal signal in which limits the framerates. However you can turn it off if you can and have some improvements and increases counts of frame rates. But sometimes the frame can be out of sync and produces artifacts and tearing and glitching. So it is normal and it is best to leave it as it is and besides you're not gonna need over 100fps as an average eye cannot detect the difference over of over 60fps.

Like for example I have my system with 2x7800GTX 256 running BF2 at 1280x1024, 4x AA and High Quality setting. With the V-sync on it gives no more than 100fps and just average on 98-99fps on single player mode. But when I turned off the V-Sync I get a artifacts every now and then on the visuals.

You having X300 that pushes 91fps that is pretty impressive but I wonder what quality settings do you use. OpenGL games basically means you can control the graphics cards settings through the games software.

Make sure you have V-sync turned off for Open-Gl. It is a different option than V-sync for Direct-X.
The Nvidia controll pannel only shows V-sync for Direct-X not Open-GL.
Try Rivatuner for Open-GL options.