In terms of monthly unique visitors here are a sample of some game sites:

Gamespot: ~12 million per month
EA.COM ~10.5 million per month
IGN.COM ~9 million per month
MSN ZONE ~7.3 million per month
NeoPet.scom ~2.1 million
Games.com ~1 million

This is from January. In January, 110 million people in the USA went on-line.