you can use r600g, which is still being heavily worked on.
I think your aged GPU is actually one of the best supported by open driver. I think its the best that can happen to an old card. Much better than no driver at all.
But if you are despirate and can cough some money for newer GPU, why don't you send the 3470 over to radeon developers? Ask on IRC.
Nexuiz is a bit choppy now when with catalyst worked perfectly (same settings)