There's only so far you could upgrade that machine. Maybe a larger HDD and more RAM, and possibly a CPU upgrade to 200 MHz (none of the "super-cpu" upgrade manufacters exist anymore), but you'd never get it running anything beyond Win98 comfortably without some serious OS hacking. Other small distros of Linux (i.e. DSL, Puppy 1.x) however may be right-at-home on such a system....
Some of the P-I motherboards could take an AMD k6 up to 550 mhz so, as already mentioned, it's going to depend on your motherboard. I'd guess though, that your motherboard with its p-90 and 16 ram isn't going to be one of those.
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