Have you fixed your problem? 'Cause i have the same monitor and the same problem: i can't set 1600х900 resolution. My graphic card is ATI Radeon HD 5770. I also tried with one ATI card without any success. But i could set this resolution on another computer with internal graphic card.
I was having the same problem a while back but just checked a system with Catalyst 10.8, and 1600x900 is now supported for my 16:9 ratio monitor. This, after a long discussion in This Thread
diverted the conversation from notebook to desktop graphics!