(rambles with myself in the hope that someone will correct my nonsense)
From what I've been reading, APC is non-distributed and is an opcode cache. Opcode caching means that the compiled PHP script is cached so that the script isn't being compiled on each request for it, it can just be read from the cache.
Memcached is a distributed caching mechanism and is used if your site is running on different servers (loadbalancing comes to mind) so the cache is available across all servers and not just the server that it's cached on (in layman's terms, it's like a shared cache - right?)
Now, once I've finished rambling I've come to the conclusion that memcached should only be used if I'm utilising multiple servers for my site, and APC should be used no matter what because it is caching scripts to save resources.
So, (on a single server installation) would I be best off just sticking to APC, or utilising memcached too for XenForo (if this is even possible)?