The number of visitors per day doesn’t really mean anything, it is the peaks that kill you. If all 2000 hits per day come within a one minute peroid, then you might have problems, but if they are evenly spread out throughout the day even on a highly computational webapp you shouldn’t have much issues.
Regardless, if you wish to scale, Varnish will probably help you the most as it allows you to set up client side caching which is as efficient as you can get as it limits the amount of interaction with your server.
APC and memcache are a fallback for when Varnish isn’t able to serve a result. APC will speed up your PHP. memcache allows you to do things like grabbing some complex data from you database for a user and then serving up a cached version of that data for users for the next x minutes/days/weeks. This can make a huge difference if you have any time consuming queries.
Edit: I’ve been meaning to try out Cloudflare CDN for a while now and after doing so I had to come back and recommend it. They have a free account (which I’m using) and setting it up is pretty easy as long as you know how to change DNS records. Out of all the technologies mentioned, this will probably be the best first step you can take to speed up your site. Just so you know I don’t have shares in Cloudflare or anything like that, but I’m seriously considering it.
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)
Because APC is so easy to put into a LAMP system, I’d put it in there (and have for my own website, and that might get 5 visitors a day), but not bother about the others unless you had some kind of problem, that required the additional effor, such as far larger numbers of visitors, or hundreds of gigabytes of image/video downloads.
Memcache would also require some active use beyond basic configuration as well (or at least using a plugin that used it appropriately, for WordPress or some other off-the-shelf software) – just installing the software does nothing at all.