Cannot allocate memory: couldn't spawn child process

[brad3260]

Hello, I'm a webmaster for a site running vb 3.8.1 and we're having problems with internal server errors. It happens when the forum gets busy and I'm getting the following message in the error log
[Sat Feb 28 05:57:56 2009] [error] [client 24.128.205.158] (12)Cannot allocate memory: couldn't create child process: 12: php5.cgi, referer: http://www.chins-n-hedgies.com/forum...splay.php?f=30
[Sat Feb 28 05:57:56 2009] [error] [client 24.128.205.158] (12)Cannot allocate memory: couldn't spawn child process: /dh/cgi-system/php5.cgi, referer: http://www.chins-n-hedgies.com/forum...splay.php?f=30

I have done everything to lighten the memory use that I can find but still having problems. Their forum is pretty busy, in the last seven days their home page alone has gotten over 20,000 hits. I have them on a virtual private server with 500MB of ram. My question is, should I need more ram with that amount of traffic?

Any help is appreciated