The 2600 is a locked CPU multiplyer, so it isnt meant to be overclocked, you can maybe squeeze almost 100mhz out of it in your bios but apart from that you're not going to be getting anyway.
You would only be able to oc a 2600 would be by changing the BCLK but thats a bit risky.
You could squeeze about 340Mhz out of it pretty easily by changing the BCLK to 110 but you really shouldn't.