In general, you can expect to need much less server memory than you would for comparable CVS repositories.
Server requirements depend on many factors, such as number of users, frequency of commits and other server-related operations, repository size, and the load generated by custom repository hooks. When using Apache, it is likely that Apache itself will be the biggest factor in memory usage. Remember to take in account other applications running on the same server; for example, repository browsers use resources too, independently of Subversion itself.