Hi fahid,
There is a huge difference, no its not true, Pentium 4 M Celerons are also available in 1MB L2 Cache, but the main difference is technology, processors core.
Celerons don't support 3d Enhanced technology, SSE2, SSE3 technology, they are only available in Northwood core.

If you are looking to buy a pentium 4 processor then make sure its core is Prescott.
Clock speed have nothing to do and its a minor thing.
For example,
Pentium 4 2.8 Processor is available for 8000 Rs, and in the other shop it will be some 23,000 Rs.
Whats the difference?
Again technology? Socket it fits in, 478, 778, 912 etc
you should google it for the in-depth information.
HT Support, Extreme edition support.
HT technology runs multi processors at the same time (2) while there are some Extreme Editions processor out that have 2X 2MB L2 Cache and show in Windows XP as you are running four processors :)