Isn't this what the xrandr utility is for? Trying out supported resolutions "manually" .. ? Then (IIRC) there's a function to make the setting permanently.
Oh, I know there's been years since you asked the question, but sometimes silly things like this just won't resolve and thread creators' tend to look the other way.
I really hope this will help you, check the man(ual) pages for further reference, or drop me a PM.