Server :: Memory Peak Caused Server Crash - Data Interpretation?

Last week my server crashed. I'm trying to diagnose the cause.

This is the relevant error message in /var/log/messages:

Code:

I'm assuming that I can conclude, then, that apache/httpd was the cause of the memory leak?

Next, I've been tracking my memory usage. Using top, this is an average memory load level for my server:

Code:

I'd like to confirm if my understanding of this data is correct, because Plesk indicates that my memory usage is only 50% or less. (Though I have read a number of reports indicating that Plesk's measurements are frequently wrong.)

Top says: Of the 2,073,156K total memory, 1,982,572K (95.63%) is being used, 90,584K (4.37%) is free. Of that sum, 421,948K (20.35%) are being used as buffers. Additionally, of the 2,096,472K of Swap, 60K is used, and 887,700K (42.34%) is cached.

My questions: Is my memory actually being 95% used? Or is the buffered quantity (20.35%) not a use of physical/virtual memory? (i.e. is it disk usage?) Does the amount of cached Swap influence the percentage of physical/virtual memory being used?

In other words, who is correct? Plesk says I'm using 40-50% of my memory, whereas top says 85-95%.