This is wrong (For AMD/ATI cards) VIDEO_CARDS="radeon" should aperently be (For AMD/ATI cards) VIDEO_CARDS="ati"
There's no such entry 'ati' for VIDEO_CARDS. Refer to `eix xorg-drivers`: VIDEO_CARDS="apm ark ast chips cirrus dummy epson fbdev fglrx geode glint i128 i740 impact intel mach64 mga modesetting neomagic newport nouveau nv nvidia omap omapfb qxl r128 radeon rendition s3 s3virge savage siliconmotion sis sisusb sunbw2 suncg14 suncg3 suncg6 sunffb sunleo suntcx tdfx tga trident tseng v4l vesa via virtualbox vmware voodoo"