@dan7 APC supports fastcgi, we've been using in with Lighttpd+FastCGI for at least two years now. It's also much more stable nowadays, and will become an integral in PHP in one of the future PHP versions. Or so they say.
Also, the purpose of APC and XCache differ from that of Memcache. They're primarily opcode cachers that run on a PHP node, and aren't sharing their cache, while Memcache is a general-purpose cache, is accessible via the network and can share its data to several servers. It also doesn't do any opcode caching, and is somewhat slower, but has more memory available in a typical install.
They are not interchangeable, but they work fine together, though. As I mentioned eariler, we're using LayerCache for layered caching of data from DB -> Memcache -> APC (could also be XCache) -> in-request local PHP array cache, where each layer is faster, but also has less memory available.