Yep, check out how much RAM and swap your server is eating up. Check all your logs. ifconfig will tell you how much bandwidth you have been using. If all else fails, upgrade your server or get someone to optimize your httpd.
Have a look to see how many apache connections there are and look at the requests, to see if they are legitimate etc. It may be that somoene is using a script that is causng a problem, a site may be getting syn flooded, first thing to do is have a look and determine whether these requests are valid or not and then go from there.