Watch out for the XP2600 as there are 2 similar versions available. There is an older version on the 266FSB and a newer one on a 333FSB. I would be surprised if the 266 version is still to be found. Best bet would be to look for an OEM XP2400 and opt for something like a Coolermaster Aero or Jet HSF. (overclock it of course!)
I built an XP2400 machine recently and it ran at XP2700 no problem.