A single redmoon instance can take up some 300 megs of ram if I'm not mistaken, so 512mb is the minimum I'd say for a winxp machine. Graphics card isn't all that important, since everything's 2D and the cpu will do the bulk of the rendering. A 3 gig HT P4 can run 3 instances with minimal choppiness, so look for something at least 1/3rd as fast as that and you should be good for a single client. Any kind of P4 will do, probably P3s too. Not sure what the minimum AMD processor would be, but a 1.3GHz athlon thunderbird was ok for me back in the days.
Your 256mb of ram isn't really enough, I hope it's running win98 or you're screwed for sure. A 400MHz p2 is pretty pathetic too, I wouldn't be surprised if certain maps were unpleasant to play. Harddisk space is irrelevant, but disks that old will probably be fairly slow, making the lack or RAM more painful (virtual memory).