Can I replace Intel Pentium CPU 1.5 GHz Processor with Celeron 2.5 GHz processor in Asus P4 (SD4) mother board?
We can't answer your question unless you post the full model number of the motherboard. Asus P4 (SD4) is not the model number of the motherboard.
Most likely a socket 478 processor. Yes, if that is the case, up to 3.2Ghz. You would need to do some research but if you're lucky you might find it. Btw, it is a Northwood (codename) processor so make sure to look for that. Personally i would get a new computer but if it means all that to you, by all means i hope you find one.