I have a notebook Acer Aspire One 722 with ATI VGA chipset on it. There are two options using propietary or open source driver. But i recommend open source driver because this applied for all variants ati/amd vga series. And the performance IMHO still looks good.
For propietary driver. You can follow this wiki (i didn't try it yet)
For open source driver:
When you install xorg, there are options to choose xf86-video-ati. If you enter all, then the driver is installed by default.
#pacman -S xf86-video-ati
Then set KMS, add radeon on modules
Ctrl+X to save, then
#mkinitcpio -p linux
To test 3D acceleration of your ATI/AMD chipset you can use xscreensaver to test 3D screensaver or install supertuxkart. If it run smoothly then your ATI/AMD is installed correctly.
Done, enjoy open source driver of ati/amd on Archlinux.