I don't know of any* quality IPS monitors under $400 that have 1440p resolution.
For gaming, the biggest question concerning IPS monitors has always been input latency and ghosting. Dell Ultrasharp monitors had the best results in these two regards during testing at TFT-Central, with Dell's 23" IPS variation having the lowest input latency not only of any IPS they had ever tested, but of any LCD monitor of any kind. They currently have a 24" monitor in your price range with 1920x1200 resolution. The acceleration tech that Dell uses on these monitors does seem to significantly reduce ghosting and makes them perform very similarly to standard TN panels in that regard. You can give it a look if you're interested, although I have no idea where you would buy one in a store:
I'd also have linked you Asus's IPS monitor, but these are somewhat known for egregious backlight bleeding. Personally I think that Dell is one of the best compromises within your price range, but that's just my opinion.