Elementary OS reboot error iMac late 2009

I need to install Elementary OS on my (late 2009) iMac 11.1. I am booting from an external USB and in when presented with a GRUB menu, I choose "Install Elementary OS" and then press e key to edit and add nomodeset and then I press F10 to boot. With Elementary OS installed, I run into problems after rebooting. First I get GRUB to go in normal or recovery mode, I use nomodeset again to boot in normal mode but instead of the GUI, I get a command line and it asks me for a username/password. After that, I run startx and get a No screens found error. I think it has something to do with my graphics card driver (Radeon 4850HD). Does anyone know how to get back to the GUI mode?