You should also check your other processes. From my experience, MySQL takes up more memory than anything else on any stack that I run. You should also implement better page caching so that PHP doesn't need to be touched when it isn't absolutely necessary. But Apache is also a memory hog that needs to be fine tuned. If you want to stick with Apache, then run Varnish in front of it.
Also, keep APC, but also add Memcached. It's much faster.
If your site has a spike-load that brings it to zero memory, then, if you can, consider launching extra instances of the server and doing a sort of round-robin (if the VPS is cloud-hosted and this is possible). If the load is constant, then definitely upgrade.
@burzum is completely right, however, that you should just switch to nginx. It's far, far better than Apache at this point. But, just to put you on the right track, quite a few people run nginx as a reverse proxy in front of Apache, and while that does speed up the server, it's entirely unnecessary because nginx can do pretty much anything you need it to do without Apache. Don't bother using Varnish in front of nginx either because nginx can act as its own reverse proxy.
Your best bet is to implement nginx with apcu (upgrade php to 5.5 if possible for better performance) and use memcached, and implement nginx's native microcaching abilities. If your site is heavy on read and light on write, then you might notice that everything is taken care of by nginx just retrieving a cached copy from memcache. While this helps with memory, it also helps with processing. My servers' CPUs usually have a 3-5% usage during peaks.
