I'm looking to make a secondary computer for my self for a different location, and I too have been contemplating the same question.
The only significant differences between the celeron and the Pentium 4 are:
P4 has 533MHz/800Mhz FSB/200Mhz host clock
Celeron has 533Mhz/133Mhz host clock
P4 has HyperThreading
Celeron does not
P4 has 1MB/2MB of L2 cache
Celeron has 256KB of L2 cache
The cheapest pentium 4 to even have the 800MHz FSB and 2M cache is the 630 (3.0) for $165.99
Unless you are getting this CPU or BETTER, my vote goes to the celeron. If you are looking to buy a P4 with 1M of cache and the 533FSB then you need to go buy a 3.2 or a 3.33 celeron and pocket the difference.
Summation
spending less than 165 on a CPU? GET A MAXED OUT CELERON
spending more than 165 on a CPU? get the best P4 you are willing to buy- and make sure it is a 2M cache/800FSB one.
**Important note**
Intel 975 and 955 chipsets are NOT compatable with 533Mhz FSB CPUs so make sure you are getting a 945 platform for these CPUs......or a 915 if you like old tech that costs as much as new tech. And FTLOG, do not buy a 925X based motherboard anymore.
Austin