Your best bet is to try some of the ‘Live CD’s’ that have the *nix install all on and running from a CD/DVD, that way you can see if you can run a larger *nix install. Linux is very un-windows like in that it needs much less hardware than the XP and later windows installs do.
Win2000 not included as it will run fine on what you have, I have a 466 MHz 388 MB box that runs all day long and many XP owners try it and think its got a 1.6~2.0 GHz cpu in it.
I have seen and used many full installs on much lower powered kit than above and things running fine with 32 MB and 66 MHz, that is why you want to give the live CDs a try first, no installing and you get to see what can pick up all of your hardware and save you the waste of install time to find something will not work with your kit quick and easy. The only problem being is that you have to download them, admittedly KDE seem to be doing their best to slow down and bloat up *nix but the less its used the better IMO.
Blackboxand a full Slackwareinstall would be fine with what you have, and that AMD pc could run any *nix install no problem.
As said Distrowatchis a good place to do some quick reading up and getting of the Iso’s.
If you really want to stay close to the windows world React OS might be what you are after