Long time reader, first time poster.
I have a problem with my WordPress installation. I even got banned from shared hosting for this and put my own server and still have this problem. Apache has a very high CPU usage (it makes 100% of 1-3 processes) on a 1.5GHz box every time I get a dynamic request on the server.
My DB is 30MB and have about 4000 pages (articles + pages) on the website.
Without cache the site works really bad (I use wp-super-cache).
I tried updating WordPress (I'm on 2.3.3. now. 2.5 blocks my server completely with a single process using 100% CPU and the older version (2.0.x, etc) work in a similar bad way), disabling all the plugins, enabling cache but didn't find a way to get decent CPU usage from Apache (MySQL works OK. Apache is the problem).
I use now an Ubuntu Server with LAMP chosen at install. The old hosting (which banned me for exceeding CPU quota) was GoDaddy. Also I should mention that accessing the permalinks page (in options) takes 30-100 seconds, time in which the CPU stays at 100%.