Use lxde if you still want what you ask for and retain openbox usability, dunno why? i think that's your only option,
Try them both out. Depending on your usage xf86-video-ati should be good enough. https://wiki.archlinux.org/index.php/Catalyst willl get you set up with fglrx in no time. Just make sure to use the right aur pkg for your video card.I for one use fglrx cause my kids like their games.