XML File replace string

I have an xml file withe a part looking like this:
<TransformationRules>
   <BIGateway>
    <SourceValue>http://doorknob/cgi-bin/ppdscgi.exe
</SourceValue>
    <TargetValue>http://grapeape/cgi-bin/ppdscgi.exe
</TargetValue>
   </BIGateway>
  </TransformationRules>

I need to find a way to replace the SourceValue with the TargetValue and replace the TargetValue's server name (in this case grapeape) with the computername the script is ran on. Does anyone have some experience with XML on how to accomplish that. I can figure the part on how to write it once is done myself, just not sure how to do the search / replace.

Thanks

set objdom=server.createobject("microsoft.xmldom")

objdom.load(yourxmlfilepath)

xPath="//TransformationRules/SourceValue"

objdom.selectsingleNode(xPath).text=yournewvalue

objdom.save(yourxmlfilepath)