The older P4 may be a Willamena core which AFAIK is a socket 370, however the 2.8 I know to be a Northwood core, a socket 478. You should be able to notice if there's a difference as the Willamena's are much bigger. If they are the same size they should work though.![]()
Make sure they have the same FSB and socket type before you swap them.
"Your computer is only as fast as its slowest component."