If I'm reading this correctly, 1680x1050 is not a supported mode for an external monitor on an x40. 1600x1200 is the only 1600 mode it supports. It's towards the end of the page, but like I said, I could be interpreting this wrong.
http://www-307.ibm.com/pc/support/site.wss/MIGR-58221.html -- James Turner http://calminferno.net
