I've found that it usually doesn't take more than Internet Explorer. Unfortunately, you don't have a choice not to duplicate system resources; you're running IE whether you want to or not. If you're finding your CPU is being hogged, the problem is with the source, not the messenger. ESPN.com's automatic streaming media content, for instance kills my old laptop. Try adjusting your settings to have control over what is running.
Another issue I've seen is certain viruses get embedded so that they are reported like a function of your browser.
I tend to have many tabs open. It regularly is using 250M and 80-90% CPU. Have to close it often, and then after closing, go to the TaskManager and kill it.
Has anyone here tried openoffice ?