Feneris
It certainly is a minefield out there and at the end of the day you seem to have a reasonably good grasp of the differing technologies.
If you want a lightweight / thin laptop got for the centrino, if you are happy to lug a brick around with you then go for the P4 3Ghz+.
If you want wireless probably better to go for centrino as you will notice vast battery life gains here, in fact the centrinos win hands down on battery life.
Dell - If you are happy not to speak to tech support then stick with them, the kit is usually OK but tech support sucks.
Media - What I see frequently now is the inclusion of a recovery disk which has a copy of the windows install disk built on to it, mostly so anyone can do a reinstall, so you will have something.
Take a look in your local electrical stores at laptops available and see what you like then go online and buy it there as you will nearly always save money. Try searching for it using one of the online search stores who will return a list of retailers and the different prices.
I use
easyvalue
I agree with above even if your processor is not top spec then get as much RAM as you can minimum 512Mb.
Intel Centrino Info
ebuyer Great deals on laptops
Good luck and let us know what you go for.