The reason I ask is because when using top I don't see a process for anything like APC. So I assume that the memory usage would be accounted for in an apache process.
Is that the case, and does that mean that the memory APC is using is replicated in each apache process, thereby taking up potentially much more memory than what was originally assigned to it?
If this is the case would memcache be a better solution, even if it's not being used on multiple loadbalanced servers?
Besides being an opcode cache, APC also provides shared memory. That strongly suggests that it has its own internal shared memory system similar to memcached.
APC uses shared memory to store its opcode cache. In the case of mod_php this memory is shared between all Apache processes. So a 30MB cache only takes up 30MB even if there are 5 Apache processes.
However, when using mod_php, each Apache process does waste a lot of resources as each process contains the PHP interpreter. Thus, when Apache serves static content (html, css, js, image files, etc) it uses a process with the full PHP interpreter loaded. To get around this, some people use FastCGI via mod_fastcgi or mod_fcgi. Using an opcode cache with FastCGI becomes a bit trickier.
There is currently no way to use memcache as an opcode cache. Even if there was, it would probably be slower than desired.