it totally depends on the install you going for...
going for straight command line interface... then feel free to dust off the old 286, looking for some GUI, give yourself about the same requirements for win95, doing decently heavy stuff, then you need just about as much computer as you do for any modern windows machine... It's very versitle like that, however, choose your GUI wisely if you go that route, they CAN hog a lot of resource if you let them...