High WordPress server memory usage

Welcome to my problem ??

We encounter a very high server memory usage for our WordPress 2.9.2 site.

Just to give you some background information:
-VPS Linux 2.6.18 using Plesk 8.6.0
-384 Mb server memory
-GenuineIntel, Intel(R) Xeon(R)CPU E5345 @ 2.33GHz
-Wordpress 2.9.2
-PHP Version 5.1.6 (memory limit 64M) and Safe Mode OFF
-Apache 2.2.3 (mod rewrite enabled)
-MySQl 5.0.77
-no traffic

Here are the memory usage stats:
-150mb: No WordPress installed.
-160mb: Only WordPress installed, no plugins, no traffic.
-220mb and higher: Only WordPress installed, no plugins, 1 visitor (me) browsing on the website (front-end).
-10mb increase for every new page I visit. It seems to be stable around 230mb (stat before).
-250mb: All 24 plugins enabled (including W3 Total cache), no traffic.
-320mb-380mb: With plugins and browsing on the website.

When using top command I see the following top7 memory users:
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
63452 apache 15 0 316m 43m 4808 S 0.0 11.2 0:03.36 httpd (WordPress website)
62749 apache 17 0 314m 42m 4932 S 0.0 11.0 0:03.34 httpd (WordPress website)
63417 apache 15 0 314m 41m 4780 S 0.0 10.8 0:02.26 httpd (WordPress website)
63455 apache 15 0 314m 41m 4812 S 0.0 10.7 0:02.29 httpd (WordPress website)
63197 apache 15 0 313m 40m 3636 S 0.0 10.7 0:02.52 httpd (WordPress website)
63043 psaadm 15 0 175m 22m 10m S 0.0 5.8 0:00.21 httpsd (Plesk)
62930 mysql 18 0 156m 15m 5276 S 0.0 4.1 0:00.50 mysqld

How is it possible that browsing on the website uses so much memory? And what is the solution?

I hope I have supplied enough information for somebody to provide me with an answer.