Low memory, possibly memory leaks?

We just released our site last week and are currently having really low memory. At time of writing this we currently have 51 mb free. We haven't run in to any problems yet, but I fear that it might become an issue if we start getting more traffic. Is that even a problem? I used to run a node server to on the same server, which took about 40 mb of RAM, so if that would be on we'd only have 10mb free. That can't be good?

Our plan is: 512 with Ubuntu Oneiric, using nginx (1.0.5) as server and php (5.4.0) with php-fastcgi and mysql 5.5.20, and memcached. php-fastcgi is using PHPFCGICHILDREN=10 and PHPFCGIMAX_REQUESTS=1000

Output from top:

~~http://f.cl.ly/items/1j0F392C0S451O260L … .17.15.png">http://f.cl.ly/items/1j0F392C0S451O260L2r/Sk%C3%A4rmavbild%202012-04-09%20kl.%2012.17.15.png" />

Below you should see a munin graph on the memory usage.

~~http://i.imgur.com/Lyuj4.png" />

I'm new to this so what I see is that the operating system "cache" is using more and more memory. I have done some searching and not really found out anything about it. From what I can see is that it's gradually getting larger, which sort of seams like a memory leak somewhere?

The issue might not be a simple fix, but I would appreciate some help in how I could better debug this and try to find what is causing this.~~~~