The 2500+ is a very old processor, I'm surprised you want to still use them, but oh well.
Anyway, the reference clock is the clock that all components multiply themselves against.
Enter the bios, find anything that sounds like it might have to do with overclocking and reference clock should be close by.
Thanks, I overclocked it to 3200+. I still use the processor because currently my money is going towards a new build, and this is the computer that I got for free about 2 days ago, and the computer before that I had to give back.