LAMP Server Performance Advice / Job Offer

I work for a consulting / hosting firm in Topeka, and I have a server 
that is periodically crashing.  The last couple of times, it appears to 
have been due to the server running out of memory.  It is a virtual 
server (on VMware ESX), so I increased the RAM from 2GB to 4GB and the 
virtual CPUs from 1x single-core to 2x quad-cores. However, it may be a 
week or more before I can tell if this has resolved the issue.

What I am looking for is advice on how to track down the cause 
definitively, and prevent it from occurring again.  (My client is 
complaining about the downtime.)  When this occurred last time (~ a week 
ago), I installed sysstat to try and track the performance, but when I 
went to look at today's report, I got a message saying: "Invalid system 
activity file".  Viewing yesterday's report works just fine, so I'm 
thinking that the hard-reset I performed this morning F'd up the report.

We would be willing to pay the right person for their expertise, and I 
hope to learn something in the process.  I'm fairly knowledgeable, but I 
think I may be in over my head on this one.

The OS is CentOS 6.3, and it is dedicated to a single website running 
the Magento e-commerce software, which relies heavily on MySQL.  Any 
advice or assistance would be appreciated.

Thanks in advance,
~ j.
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