Where are web  Parts stored? I have a five part  answer:

1. Web Part metadata related to a list or page  and its sequence in a
given Web Part Zone is defined in the Web Part table of the WSS content
database. If any built-in or custom properties have been altered there
will also be data in the Personalization table.

2. Web Part DWP files are placed into various locations depending on
what gallery the Web Part is listed in. If listed in the Team Site
gallery, the DWP is stored in the top-level site's Data Part Gallery. If
listed in the Virtual Server, the DWP is stored on the file  system in
the IIS home directory \wpcatalog subdirectory for the IIS Web site. I
don't think that the DWP is kept anywhere if the Web Part is only
imported into the page.

3. The Web Part DLL and any class resource files are stored on the file
system in the IIS home directory \bin subdirectory if deployed locally.
Or it could be placed into a privitePath defined in the web.config. If
deployed globally, the Web Part DLL is stored on the file system in the
GAC and global class resource files are stored in the \wpresources
directory at the same level as the 60 directory in the "60 hive". In
addition, if deployed as a Web Part Pack, the CAB file may also be
stored in the "60 hive" WPPACK subdirectory.

4. The Web Parts that will be initially placed onto a page when a WSS
site is created are defined in the ONET.XML file's Modules section of
each site definition. Look for <view> tags for Web Parts that will be
included, like this:
<View List="104" BaseViewID="3" WebPartZoneID="Top" />

The Members Web Part (and the example  you gave below) are inline Web
Parts:
<AllUsersWebPart WebPartZoneID="Right" WebPartOrder="1">
<![CDATA[
<WebPart xmlns="http://schemas.microsoft.com/WebPart/v2">
<Assembly>Microsoft.SharePoint, Version=11.0.0.0, Culture=neutral,
PublicKeyToken=71e9bce111e9429c</Assembly>

<TypeName>Microsoft.SharePoint.WebPartPages.MembersWebPart</TypeName>
<Title>Members</Title>
<Description>Use the Members Web Part to see a list of the site
members and their online status.</Description>
<FrameType>Standard</FrameType>
<IsVisible>true</IsVisible>
</WebPart>
]]>
</AllUsersWebPart>

5. Locate other built-in Web Parts found in SharePoint DLLs here:
http://[sitename]/_layouts/1033/NewDwp.aspx

Or
http://[portalname]/_layouts/1033/NewDwp.aspx

Or even,
http://[portalname]/sites/[sitename]/_layouts/1033/NewDwp.aspx