What motherboard do you have. Features in BIOS will determine. Locked multiplier AMD's I have overclocked by increasing the FSB speed on my RAM such as on an older system I went from 200 Mhz to 220 Mhz for a 10% overclock on a system with DDR 2 800Mhz RAM. I didnt have to adjust voltages at 10%. But my motherboards BIOS has the ability to overclock. Not all motherboards have this, specially less likely to have this feature if its a name brand system that you bought taht your trying to overclock such as an HP or emachine etc which manufacturers usually remove the overclock features from a board that would support it, and sometimes a BIOS flash can add the functionality back to be able to overclock etc.
This overclock brought a 2.3Ghz CPU to 2.53Ghz increasing my RAM's FSB from 200 to 220. CPU ran about 8 degrees hotter which isnt bad for the 10% performance gain.
How much of an overclock are you looking for. You might be able to get 10% to 15% without messing with voltages, but you have to watch out for heat as a byproduct of pushing it harder, so you will want to keep an eye on CPU temp. On my build 12% was about the maximum I could push my locked multiplier AMD before it started to show signs of struggling and instability, heat was also warmer and so I decided to back down for greater stability vs random blue screens and choppy graphic issues as the CPU was stressing. Back down to 10% where choppy graphics and blue screens weren't going to happen. My CPU was a AMD Athlon 64 x2 4450B 2.3Ghz with 1MB Cache ( Business Class Dual Core CPU ( B = designation) ) and 45 watt TDP. Overclocking it, it consumed more electricity and forced this CPU to operate as if it was a 65 watt TDP CPU. Motherboard also gets stressed through overclocks sometimes such as VRM's (Voltage Regulators ) run hotter for CPU.
Also to mention.... RAM quality will matter too when messing with increasing the FSB. I have gaming matched pair Corsair DDR2 XMS2 sticks with heat spreaders.
