Some 486 machines may be fine if your uses for it are limited. Continue reading to make a more informed decision. In both cases, the more or bigger, the better. If you plan on running games or a lot of newer software you should plan on expanding it to 64MB as soon as possible. Hard drives are quite cheap.
Some older machines use different memory chips than newer machines.
In this case, older can mean more expensive - MUCH more expensive. Also, increasing memory may actually mean tossing out the old and replacing it with new, not just adding to the old. If in doubt, contact the machine's manufacturer for specifications. Even if you don't have a lot of hard drive space or memory, there are software packages available that don't have the giant appetites of the major software vendors. If you're a real hot shot and want to play a lot of games, don't go for any machines less than a Pentium III or Celeron.