Linux Server eating all the memory

I run all of my websites on a 512MB dedicated virtual server, which up until now has served me very well.

On 10 Jan, for some unknown reason my memory consumption went up from averaging around 50% to 70-80%. To my recollection, I did not make any changes to the server that day and none of my websites have been receiving more traffic than usual, so technically there should be no reason for the higher memory consumption. The only possible reason is a 'yum update' which will have installed some updates of which may be eating more resources then they should be.

Nevertheless, I decided 3 days ago to try to optimize my server and reduce its memory consumption. I did this by following the steps in this link:

Making it better – Media Temple

I succeeded with all of the steps, except the FastCGI PHP module for complicated reasons. Regardless of this, the steps have had the opposite effect and I now seem to be using over 100% of my memory most of the time. Please note, the reason why I am able to use over 100% is because the server actually has 1GB but I am only meant to be using up to 512MB.

I have tried rolling back the changes I made by restoring the back conf files I made, but this has had no effect.

I did run another 'yum update' which could have installed a resource eating application

For your info, the mirrors that I use for 'yum update' are the default Plesk ones and Atomic. This might be relevant as the problem may be coming as a result of this not what I have done.

I also ran 'up' for the micro updates, which again, may have caused this problem

My questions are:

1. Is there a plugin to see exactly what is eating my memory?
2. Are the steps in the aforementioned link reliable in the sense that if configured correctly they will consume less of my servers resources?
3. Is anybody else noticing higher memory consumption from the recent Plesk and Yum updates? My Plesk version is psa-10.3.1-cos5.build1012110718.17