Server starts to time out with only a 100 users on it.

So I have WordPress site running on a stock LAMP Stack (apache2, mysql, PHP 5)hosted on a Medium AWS EC2 instance. The database is running on a RDS instance and I’m using W3C Total Cache. Anyhow, the site is running smoothly except for one problem. Lately when we have about a 100 or more users (According to Google Analytics) on the site at a time, the server becomes unresponsive and times out.

I’m looking for some basic tools to help me trouble shoot and possibly suggestions for what to look for.

I know the server should be able to handel many more users then that.

Also what would be the recommend http.conf settings for a server like this. I need it to handle at least 1000 requests or more at once.