Manual:$wgMainCacheType

Cache: $wgMainCacheType
Object cache setting
Introduced in version:	1.5.0
Removed in version:	still in use
Allowed values:	One of the CACHE constants (see below)
Default value:	CACHE_NONE

[edit] Details

Object cache setting. Replaces $wgUseMemCached.

Here are the available CACHE constants, as defined in Defines.php

CACHE_ANYTHING - Use anything, as long as it works
CACHE_NONE - Do not cache
CACHE_DB - Store cache objects in the DB
CACHE_MEMCACHED - memcached, must specify servers in $wgMemCachedServers
CACHE_ACCEL - APC --> XCache (--> WinCache, if on Windows), trying each in this order, using whichever is available

As of MediaWiki 1.18, current versions of eAccelerator no longer work. In MediaWiki 1.19 support for eAccelerator has been removed.

CACHE_DBA - Use PHP's DBA extension to store in a DBM-style database in $wgTmpDirectory with $wgDBAhandler

- Roan Kattouw has stated that CACHE_MEMCACHED directive will provide the most significant performance improvements in the case where you have memcached and an opcode cache installed. APC still caches ( others may as well,) though I am unsure it is used in the final transaction with the client.
- This is just one of MediaWiki's caching settings. For best performance you may need to consider other caching settings, such as $wgParserCacheType. See also Squid or Varnish which can help a lot.

If using CACHE_ACCEL with APC, increasing apc.shm_size may improve performance