Macbook Pro 15'' early 2011 - external monitor 27'' with DVI connection
If you want a resolution of 2560x1440 and you want to use DVI, then you must use a Dual Link DVI adapter.  Period.  A Single Link DVI connector will only support up to 1920x1200.

That said, I wouldn't recommend going with Dual Link DVI but rather getting a monitor that natively support Display Port or mini Display Port like the Dell UltraSharp U2715H



I am a big proponent of the idea that the less you have to convert, the better the reliability.