the EUFI BIOS for the M5A97 is pretty tricky (I own one actually right now, in the computer I'm typing this message through). You have to hit the exit button in the top-right corner, and then select the tab "Advanced". Then, make your way to the A.I Tweaker. There should be a ton of options for your X4 955, and then begin to increase the CPU Multiplier (Or FSB). Once you've seen to become unstable after every adjustment, increase the Vcore step by step till the most recent overclock is stable. Then, continue to bump up the multiplier/ voltage till you seem satisfied with the performance.
(I'm currently running my Athlon II X2 270, at 4300Mhz/1.5V)