have you tried CPU-Z? Anyway, I think that possibly your BIOS has set the wrong "Front Side Bus" speed. The speed you quote is suspiciously close to half the the proper speed... Did you go into the BIOS after installing the new CPU?
It could be that your 2.6 GHz part had an FSB speed of 100 MHz, whereas the 3.0 part would need a 200 MHz setting.