Response time isn't the most important stat for monitors when it comes to FPS games. Input lag is the most important thing to look at, the 120hz monitors offer the best input lag. One of the best is the Alienware AW2310, it has zero input lag. If you want the best picture quality then you will want an IPS monitor, which have much higher input lag and response time.
Edit: Since you're looking at 27" monitors you're SOL. There is no 27" monitor with zero input lag.
GTS 250 on a 120hz screen?