Hi nguyentrung13,
I'm not an hardware expert, but yes, you can overclock your GPU for getting a better score, but you could also lose in stability, especially in long rendering sessions.
If your GPUs have hybrid or water cooling, you can gain a bit further.
Please, have a look at this other discussion for a direct example:
viewtopic.php?f=11&t=45807&hilit=overclocking&start=10#p229922ciao beppe