I've experienced this myself with an Nvidia card and an ATI card; I guess
some of them just suck, not surprising since AMD doesn't fully cooperate and
Nvidia doesn't cooperate at all with free software.
The obvious thing to try is a kernel update to the latest version of
Linux-libre. If that doesn't work, you'll just need to either get a new card
(http://libre.thinkpenguin.com sells a good one) or deal with the wrong
resolution and hope it gets fixed in the future.
By the way, I don't see your card on h-node.org, so it would be cool if you
could report it. :)
