That means the only upgrade there is is a Pentium 4 CPU. It would be faster than Celeron one but sluggish compared to any decent CPU on the market (if BIOS accepts your new CPU and if you manage to get one in). Here is what I have found:
Pentium 4 is 2x slower than same frequency Pentium D
Pentium D is 2x slower than same frequency Pentium Core Duo
Pentium Core Duo would be 2x if not more slower than current Intel i3 offerings
In other words you can be looking at 16x or more in raw power increase from P4 to Intel i3.
IMO time to upgrade. Pentium 4 is generations back. Celeron is just a handicapped version of one.