I've been wanting to do this for the longest time... ever since it started becoming a real pain in the @$$ to have to wait 2-3 seconds everytime I wanted to read a new incoming email, or a new newsgroup posting.
I am assuming it has to do with how Outlook organizes its .pst file - all the entries in your Inbox and local folders are kept inside the one .pst file, in my case resulting in a behemoth of a .pst measuring almost 1.3GB in disk space.
I finally had enough and moved all my stuff over to Mozilla Thunderbird. I am a HAPPY CAMPER now! No longer do I have to wait, no longer do I need extra plugins for RSS feeds and NNTP news reading and most of all, the CPU usage is now negligible!
Oh- and for those Outlook die-hards who can't have it any other way: There's a neat Theme (skin) for Thunderbird called Outlook 2003 SilverTB that makes it look JUST like my Outlook looked before!