This section provides a sample working config from an AutoComplete field.
<AutoCompleteProperties>
<URLToXML>http://hostname/site/Forms/Tasks.aspx</URLToXML>
<DisplayValue>
<values>
<value type="xpath">NewDataSet/Table1/Title</value>
<value type="text"> assigned to </value>
<value type="xpath">NewDataSet/Table1/AssignedTo</value>
</values>
</DisplayValue>
<DataKeyValue>:xpath=NewDataSet/Table1/ID</DataKeyValue>
<ValueMember>:xpath=NewDataSet/Table1/Title</ValueMember>
<UpdateFields>
<Field>
<Title>Author</Title>
<FieldValue>:xpath=NewDataSet/Table1/AssignedTo</FieldValue>
</Field>
</UpdateFields>
<EnablePaging>True</EnablePaging>
<ItemsPerRequest>5</ItemsPerRequest>
<EnableLoadOnDemand>True</EnableLoadOnDemand>
<MarkFirstMatch>False</MarkFirstMatch>
<IsCaseSensitive>False</IsCaseSensitive>
<LoadingMessage>Loading Tasks...</LoadingMessage>
<Skin>WebBlue</Skin>
<BusyMessage>Adding Tasks</BusyMessage>
<CreateNewItemUrl>http://hostname/site/Lists/Tasks/NewForm.aspx</CreateNewItemUrl>
</AutoCompleteProperties>