I wouldnt consider any AMD card with less than 800 shaders for gaming purposes. Spend the money and get a card with at least 800 shaders.
I run xfce4 as my desktop system on a very old gentoo installation. It is fully updated, but the time that I originally installed was a looong time ago. I've got a lot of cruft built up.
EDIT: It is still true, even in todays day and age, that when buying hardware for linux you have to check compatibility first. For now the r600g driver is better than the radeonsi driver, so get something that can use r600g.