Some people have said that using the signature file included with RBE on their 4850's didn't work. I had a problem with it too, but through persistence I found out a way around it.
1) Before flashing the 4850 with the new signature file, turn off Overdrive in ATi's CCC.
2) Flash with the new signature.
3) On reboot, DO NOT ENABLE OVERDRIVE! (This would mess up my display, even if the overdrive values were set to stock)
4) Use the AMD GPU Clock Tool to change the GPU and MEM frequencies to reach your maximum clocks.
5) Use RBE to write your maximum values into the BIOS.