I don't think it will cause you to lose fps. If you are running the GT120 and the games at highest setting, that is your problem, not the LCD. If you get a standard LCD with a 60 or 75 Hz refresh rate, and your games fps is already higher than that, you wouldn't notice anyways... the human eye can't keep up over those refresh rates anyways. If your games are lagging, tone down the settings a bit or change the resolution. I still run a 9800Gt in my q8300 system... I turn off AA and set the resolution to 1280x720p or 1600x900 for most games and the quality settings are still on high and most games are over 60fps anyways. Even if your game and GPU can achieve 120 fps, a standard LCD can't refresh more than 60 fps anyways.
Also, sometimes when using the d-sub connector at high resolutions, you may get tearing, but switching to DVI will usually fix it.
In your case the LCD has nothing to do with lag. Either tone down the resolution and turn of AA, or change your video card.