your eye might notice the difference at some conditions, the problem is that your GPU will not be able to render at that speed and your muscles will not be able to react to anything that your eye notices in that time frame.
Hi. Personally, I'd go with the 24" Dell U2412m ultrasharp you listed but I'm not sure you'll be able to find it for under $300 from a reputable dealer. I'm not familiar with provantage, who according to your link are selling it for $280. It's currently on newegg for $339.
Dell makes some excellent monitors. I purchased my first 24" Dell ultrasharp some 4-5 years ago and haven't had any issues, would purchase another one in a heartbeat. Either way, I'd strongly suggest getting a 24".