my machine also has 1 gb of ram and i loaded kde4
i know that kde4 can work with a 1.2 ghz cpu decently, not fast but not really slow
i'm sure that if i still had my old amd duron at 950 mhz i could run kde4 there too but probably slower than i wanted it
lately the limitation on what you can run on what hardware is the video card and the cpu technology
a couple years ago the kernel supported pentium II but if i remember correctly the current kernel doesn't support it
some old video cards are abandoned by the manufacturers and no official drivers exist for them anymore, or never existed and that is ok, the problem comes when xorg stop supporting them too, i have one in that situation, lots of workarounds to make it work as it should(or closer to that)
other than that, you can work with a old 20gb ide hard disk and old pc133 ram if you have it, it will be slow but usable
sometimes what you should ask yourself is if you should keep using your time trying to revive old slow hardware or just recycle it and get something more contemporary, form he last 5 years, not more than 10