I had a similar resolution problem with Ubuntu. I wasn't sure if it was because of the monitor I've got or what.
I've got a U-Plus Vision 30" monitor - which I can use at 2560x1600 resolution in Windows, if I uncheck "hide modes that this monitor doesn't support."
I've tried everything I could think of, and couldn't get it to go higher than 1280x800 with Ubuntu...
Here's the thread I had at the Ubuntu forums.
I hope that helps.