Upgrading CPU in a Laptop

[ComputerWizKid]

I have an older Laptop computer that I want to up grade the CPU in (Currently has a Celeron M 2.8 GHZ 400 MHZ FSB) The CPU is a standard socket 478 so I do not have to worry about a soldered on CPU like In some laptops. I found a P4-M on eBay for $25 but its 800MHZ FSB can I use that?or do I need to find a 400 FSB Chip? and would a celeron to a P4 be a big improvement in performance or what other upgrades can I do to this computer?