That graphics card (or any ATI for that matter) will do pretty much ANY resolution you want.
Powerstrip is your friend here.
It’s just a matter of getting windows to accept it. It usually involves setting the custom resolution on BOTH screens. You don’t have to use it on both, just accept it as a resolution on both.
If you don’t windows has a habit of ignoring it.
The reason for not supporting HD resolutions is that no timings was defined for it by default, most drivers solves this by adding their own resolutions into windows (with the right timings, or at least a timing at all). At least I think it’s the reason.
So updating the driver is recommended as a first test.