Sapphire R9550 ATI R9550 128MB
The R9550 VPU should have a good potential for overclocking, as it's based on the R9600 and is artificially clocked down to 250MHz. In theory, we should be able to clock it to the default R9600 speed without any problem. Sadly, all R9200/9550 VPU are locked since Catalyst 3.4. The only way of overclocking is to use a hacked driver or firmware.