I just want to know if someone has tested if an external monitor works at 2560x1440 with an iMac 21.5" Late 2009 connected via Mini Displayport - Displayport cable. I can't find anything certain on the web.
Apple says it does:
Simultaneously supports full native resolution on the built-in display and up to a 30-inch display (2560 by 1600 pixels) on an external display
A well made Mini DisplayPort to DisplayPort cable should work just fine.