i would guess they are different cores, meaning that more than likely the 1.5 will not work unless you flash the BIOS to support the core of the 1.5...which in turn probably isnt all that likely, because Dell doesnt have the greatest BIOS updates, but you could still check it out..my guess would be that the 1.5 is the Willamette core, whereas the 2.26 is a Northwood, but i cant guarantee, you can run CPU-Z on it, to see what kind of core it is, as well as other interesting stuff. but just some stuff to think about.