FIX: You experience high memory usage in the W3wp.exe process on a Windows Server 2003-based computer that has Internet Information Services (IIS) 6.0 installed
This is now an important issues with those are working around in windows iis6. Microsoft have reported the heavy memory usage in w3wp.exe worker process is due to transfer of a large HTTP response that is generated by an Active Server Pages (ASP) application to a client computer. An example of an ASP method that can generate this large HTTP response is the Response.BinaryWrite method. The memory usage in the W3wp.exe process may be as high as the size of the whole HTTP response that is being transferred. Typically, this problem occurs when the client computer uses a slow connection link.
You will get more info regarding this issue directly from microsoft kb
http://support.microsoft.com/kb/916984#top