The Celeron is more of a budget orientated cpu it really depends as to what you will be using the computer for and what your expectations of its capabilities as to whether it will matter if you purchase a Celeron cpu or not.
Here go to this
Link this should give you a much better idea of whether it is what your after or not, If your not buying one it will still give you a better understading of the difference.
Edit: I recently had to wipe a mates sytem with a (p4 3Ghz cpu the rest of the system wasn't poor quality parts either) and reinstall everything and going from what was in my sig I found it slow and slightly irritating