NO 120Hz is FUCKING garbage until they make it work on IPS panels.
TN panel = garbage in terms of color....
Ask yourself,, what's more important... COLOR, or refresh rate...
Well, given that he's currently using a TN, I would figure he already chose something other than color.
TN is also pretty popular, because there are many of us who just don't care about color accuracy. Yes, I said it. TN is all I'll ever need, unless I get into a professional field that requires better, and pays well enough to justify a proper pro monitor at home. Not only that, but I find it much easier on the eyes for reading text than IPS or PVA, which is something I do a lot of. If I only have one monitor, it will be a TN, regardless of budget, given current technology.
Then as yourself,, WHICH card can do 120fps at 1920x1200 consistently... NONE
Well, there's more going on, there. First, you don't need constant 120FPS, you just need constant >60 FPS (doable with older games, and some newer games at reduced details). Secondly, the image appearing sooner on the display, and more images in some small period of time, improve the perception of motion and game environment details, since no game that I know of has been able to effectively implement motion blur (for one thing, the game engines would need to be able to predict how long the current frame will take to render, for it to be truly effective).
Is it worth 400 CAD? That's very good question, since the OP already has a pretty good 60Hz gaming monitor, and both monitors are the same resolution, and a short res, at that.