Hi, welcome to LQ,
I have a Radeon 5470 in my Laptop. I've noticed that since some weeks the opensource-driver (module radeon) is better then the proprietary one.
I would recommend that you uninstall the proprietary driver (you could blacklist fglrx and rename the xorg.conf file which has been created while installation (BTW: did you run aticonfig after installing the proprietary driver??? (aticonfig --initial))
Then after booting you could check which module is loaded, as root execute
lspci -k | grep -iA3 vga
this should show you some information about your graphics-adapter and which module is in use.
As of the proprietary driver, it uses an xorg.conf file, you could edit this and check which resolution is configured.