People view web pages on monitors of many different sizes and resolutions.
Many people don't even realise that you can scroll down a web page let alone change the resolution of their monitor, so it's important to make sure that the people who are going to use your site at least get to see what you want them to. Monitor size and resolution is an important consideration. In order to cater for the majority of users, it's still a good idea to design for the 640 x 480 size. Around 480 pixels across is a fairly good start for an opening graphic. There are also some differences between viewing graphics and text on Macintoshes and PCs.