I agree with XinoXide, get a 1440p monitor that clocks at 120hz. However, if you do not want to go down the Korean IPS display route, the monitor you choose depends on what games you play. On my dell, when playing BF3, I can notice the input lag and after coming from a 120hz benQ, it is quite noticeable. That said, this is only noticeable when playing online, I think the added latency of being online combined with the input lag from the monitor causes this. When playing bioshock for instance, I don't notice it at all.
Also, I am one of those people that do notice that difference between 60 fps and 120 fps. The difference is subtle and is only noticeable if you have been accustomed to it, its not about seeing those 120 frames but more about how it feels. But hell, it could be a placebo affect. Unless you play online shooters a lot, I would go the 1440p route.