CPU high, I'm being a lousy naighbour

Hello everyone, I'm running on a 2048box with CentOS 6, Apache, nginx as static files proxy for a Wordpress built site with APC turned on and W3 total cache.

You would say - the dream team, but in times of high load the CPU is pretty much high with some 150 - 250% load, so I don't feel like a nice neighbor.

I made a standard test using loadimpact.com (1 to 50 concurrent users) and the outbound traffic went as up to 4,5M and the CPU spiked at 240%.

I would really appreciate some help in finding the reason for this behavior since it doesn't seem normal.

Oh yes, "top" tells me that each Apache process starts with 32MB and then gradually goes as high as 55MB.