Is it bad to overclock the CPU?

Aspire
System BIOS version:V3.1
Pentium II
Speed: 266MHz (overclocked-300MHz)
L1 Cache size:32KB
L2 Cache size:512KB
128MB RAM
4GB Harddrive
OS WIN98

It has extra cooling fans, but now I just want to know if I can install a faster CPU andhow. Any help is appreciated, thanks.