The claims include that IE8 has a memory footprint approaching 400MB, across six processes and 200 threads. The end result is that the browser is more taxing on the system than IE7 and Firefox. It also apparently consumes more system resources than Office 2007 programs.
The browser is still in beta, so there is still a long road for Microsoft to travel down and make changes. Hopefully they’ll examine just how heavy the browser is. Whether or not modern desktops come out of the box with 2GB, 4GB or even more, people want their browser to be lightweight and simple. At least, that's what Google thinks, with the recent announcement and introduction of the Chrome web browser.