Hi, poster, I am a Mint 13 maya user too.
After buying a second handed ATI Radeon 9550 Video Card, I am now facing the same problem as you were.
The resolusion is up to 1024X768 at most and I am not satisfied with that.
I have tried to downloaded the driver from AMD official site, the version of it is 9.3 with the name of "ati-driver-installer-9-3-x86.x86_64.run",
Then I typed "sudo sh ati-driver-installer-9-3-x86.x86_64.run", finally it showed that
"Error: ./default_policy.sh does not support version
default:v2:i686:lib::none:3.2.0-23-generic; make sure that the version is being
correctly set by --iscurrentdistro
Removing temporary directory: fglrx-install.rdPhCs"
It was quite a frustration.
And I searched and found your post, still not get any clue of solution.
If you have solved this problem, would you please let me know how?