External Monitor Resolution Problem

Hii

I've a VGA monitor which has max. resolution of 1600x900. The problem is that, when i connect this monitor, there is only two resolutions i can select. 1024x768 & 800x600. the monitor shows as "Unknown Display".

So, everytime when i connect, i had to

1. create the modeline for 1600x900
2. add the modeline to VGA1 via xrandr.

Is there anyway to resolve this? And if i booted up the PC with VGA connected, it shows some display output error.