In my ze5600 (ze5620us to be exact) laptop, I have a Celeron GHz processor, and would like to upgrade it to any form of Pentium 4. Would this be possible? And if so, what would be the best pentium 4 to choose for my laptop?
If you're wanting to do a CPU swap, I would first warn you that it isn't a job for the faint of heart. There can dozens of little screws in the laptop, of different sizes, to track. Easily torn ribbon cables are also inside. If you've never done this before, you do -not- want to practice on a machine you rely on daily. The speed gains you will find are marginal at best.
If you choose to proceed, you also may need to modify your cooling a little to deal with the extra heat. Then there's the whole issue if you've got one of the boards based on the Pentium 4 or Pentium 4 M (which consumes less power, but is still a space heater compared to modern chips).
The answer is technically "yes", but is it practical or advisable for what gains you'll get? No. You'll still be stuck with a 2003 - 2004 ish laptop that really won't run much better. For the money you'll spend it really won't be worth it.