First,in the BIOS set the RAM clock on FSB level(ratio 1:1) and lock the AGP/PCI frequency on 66/33 Mhz
Raise the CPU core voltage from 1.38 v to 1.50 v.
Increase the FSB clock from its standard 133 Mhz speed in 10 Mhz steps,each time testing the stability of the system under full load-the CPU temperature should remain below 65'C
and to check performance increse use benchmark such as everst or sisandra