Screen resolution messed up after installing Gnome Shell

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http://askubuntu.com – It's been hell getting Ubuntu 12.10 installed, but I've finally got it working. I'm using nomodeset, and I have an AMD Radeon HD 6540G2 graphics card. Last time I tried to install fglrx, it messed up everything and it kept telling me it was booting in low graphics mode (I ended up reinstalling Ubuntu to fix it). Now, when I was using Unity the screen resolution was fine, but I've just installed Gnome Shell and the screen resolution is 1024x768 (it should be 1366x768). How would I fix this? (HowTos)