120hz computer monitors require two dvi connections to run. both dvi output ports on your card will be used to power one monitor.
in general i'd say 60hz monitors are better as you can get a better quality panel (va, e-ips, p/h/s-ips) instead of low grade tn type panels.
60hz is pretty much the standard refresh rate in the usa. unless you're running 120fps in a game, having such a monitor is a moot point.