The server loads all message objects into memory during load (read) and save (post) operations. This means that you need enough memory to handle the size of the biggest message you plan to be able to handle. For a simple BBS, 256 MB of memory might be just fine. On the other hand, if you're running a big groupware system for an organization, you'll want to add enough memory to support the number of concurrent users you expect to have logged in. For example, if you have hundreds of users, you should have several gigabytes of memory. Also keep in mind that the full text index consumes a lot of memory too, so plan for a little extra if you are going to enable this feature.