Yes. There would be a difference, depending on how much you up grade. For example, a Celeron 400 runs circles around a Pentium 200.
As for memory, I like 128 more than 64, but I run w2k.
You need to tell everyone what motherboard you own, the motherboard (and BIOS) determines what the fastest Celeron is that your system can handle, so what is the mb and chipset?
You may want to consider buying a new motherboard. Some people on this board boast about fast and cheap Duron systems. That could be worth looking into.
Good luck.
Post all details on your system, and what you use the system for.
jeremy806