P4 2.8E and P4 2.8C What is the difference between

845PE can be on these two CPU you?

That depends on the support of the board of the FSB, the main board and a brand, Your board last P4 2.4 is better.2.8c package is the FC-PGA2, manufacturing process 0.13UM, core type NORTHWOOD, 512KB secondary cache
2.8E package is PPGA, manufacturing process 0.09UM, core type PRESCOTT, 1MB secondary cache
support both Hyper-Threading Technology

you look at 800MHZ board does not support, does not not work on