take the time to read through a few other posts just like your own that have been responded to. we often go through a whole list of things to help you pick out what you want.
things we discuss:
60hz vs 120hz
sizes and resolutions
so please at least try to look around first to get familiar with at least the lingo.
to provide a quick answer: unless you are getting 100+ fps 120hz is useless. that said, some people find it "smoother" but figure you are still using a TN panel which is the lowest end of the spectrum. with your budget i'd suggest an E-IPS type since although its still a 6bit panel like the TN it also features a near 180 degree viewing angle without color distortion which is a bad thing about TN panels. response times are a bit higher and its only 60hz but this is fine. the only reason i can see going for 120hz is if you want 3d.