Can't run 1080p with Radeon 6870 on Ubuntu 12.04

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http://askubuntu.com – I have a XFX Radeon HD 6870 and a 1080p monitor that works fine in Windows, but in Ubuntu 12.04 with the ATI/AMD Proprietary FGLRX Driver (post-release updates) installed I can't select 1920x1080 as my resolution. The same occurs with the open source drivers. Any help would be appreciated! (HowTos)