This is a super question. I also am interested in upgrading my PC via new cpu. I will add this post to try to get a busy Intel person to take note. My mobo manufacturer says my mobo only supports up to E4500, with LGA775. The E4600 cpu has LGA775. The E4700 has PLGA775. So, I think maybe these little tiny differences are details...and the devil is in the details! I think maybe safe course to avoid wasting money is to go with every tiny detail...e.g., LGA775 is NOT same as PLGA775.
I had the same problem a few months ago when i tried to replace an old pentium D with a new Duel-Core processor. Mmy original processor was PLGA775 the new one was a LGA775. After swaping it out i also got an orange light and no responce; so asked a few forums i belong to and they told me that PLGA775 was the predecessor of the LGA775 socket and that although some CPU's may fit both sockets, it shouldn't be assumed unless stated in the description.