Like most everything else: it depends.
If you have a lot of calculations and can store the results then caching will speed things up. If you're just basically storing rows from the database then in memory caching will help but memcached may not add a huge amount of difference vs. a database (assuming the db queries are all simple). On the other hand if you're doing complex queries, or a lot of programmatic work to create something, then caching makes much more sense.
To give you an example, I recently worked on a site that was written by a 3rd party contractor who did not do any performance work during design. It was slow as an ox because it had a lot of unoptimized includes and such. Adding APC basically improved the performance by 10x. Adding memached decreased load times by 10 - 20 ms.
If you're far enough along then do some performance testing (look up xdebug, or another tool) and see where your bottlenecks are, then plan accordingly.