Cache

Cache:

1. APC
2. Varnish
3. memcache

combination of all 3 is useful but use them for different things: Varnish: can cache static content and deliver it extremely fast (reducing load on apache)

APC: stores php opcode so that calls which are processed by php are faster

Memcache: use as a temporary data store for your application to reduce calls to your db (db is typically a bottleneck)
if you have time on your hands, go for it with all 3 in the following order:

APC (fast to get up and running)

Varnish (needs a bit of configuration but is well worth it for static pages)

Memcache (code changes to make use of it, so obviously needs more thought and time)