You might be able to overclock it with ATI Tool:
http://www.techpowerup.com/atitool/
Be warned, this will void your laptop warranty and could damage it if you are not careful.
I would start with a 25 MHz core overclock. You don't have any dedicated memory, so once you have a 25 MHz core overclock, run the artifact test with ATI Tray Tools from my sig. If it doesn't find artifacts, then repeat the process untill you have a 100 MHz overclock. Then run the artifact test and game for a couple days and see if you have problems. If you don't, you can try overclocking another 50 MHz. I wouldn't go past a 150 MHz overclock.