Simlilar issue with radeon 9600 agp.
I use kernel 3.6.8

In logs:
dmesg: Forbidden register 0x4F34 in cs at 40 (val=00000640)
radeon_cs_ib_chunk] *ERROR* Invalid command stream !

triggered by opera-next (HA,webgl enabled), just scroll down any page and it
hangs.