120hz monitors are usually marketed as "first-person shooter monitors" and for good reason. When a game requires lightning fast reflexes, getting a frame every 8ms vs every 17 makes a big difference. Right now I have a Dell 24" IPS next to a BenQ 27" 120hz TN, and while the IPS is easier on the eyes, it just feels choppy and unresponsive after using the 120hz panel for a while. It's really hard to go back to 60hz once you've known better. And I haven't even tried the LightBoost hack yet (google it!).
120hz IPS panels exist, if you're ready to deal with Korean sources directly.
What model is your Dell 24", if you don't mind me asking? Just curious.