Still learning about systems but I know a lot about CoD(UO)
If your frames per second max out at 90(91) you have probably set the max. FPS yourself.
The default is 60FPS but when you put the option"Sync every Frame" to "off" your framerate will rise to whatever your system can do max.
In this case your FPS fluctuate a lot which is not preferable for good gaming, for example:
If your FPS varies between about 100 and 200 you will possibly get the best performance if you lock the max FPS at 125.
You can do that ingame by bringing up the console(~ key) and typing:
/seta maxfps 125(or any number you like)
Now your FPS won't go higher than 125 which is good because anything higher than 125 you will not notice ever.
Now your system does not have to render any more frames than 125 and thus has a slightly easier job, this often results in higher minimum FPS.
In the end you can very well end up with a game running always at 125 FPS which is the best in my opinion.
(If at 125 the FPS still fluctuate you can go lower like the 90 you have now but with your current videocard I'd go for 125 that's the ulitmate FPS if stable)
Shooting the enemy with a little connection-lag is difficult enough so anything you can do to get less video-lag(or fluctuating FPS) is a good thing.
Come play at the GITS servers someday:
http://www.gitsclan.co.uk/