while copying the sharepoint 2010 list items through SPListItem.Update(),it updates the SharePoint built-in metadata fields like Modified Date, Modified by, version etc with current date etc..

SPList list = oweb.Lists.TryGetList("ListName");
SPListItem listitem = list.Items[1];
listitem.Title = "Test List Item"
listitem.Update();
list.Update();

But SPListItem.SystemUpdate() method in the SharePoint 2010 object model updates only the custom metadata fields and not the built-in metadata fields. See the code below for using SPListItem.SystemUpdate() method

SPList list = oweb.Lists.TryGetList("ListName");
SPListItem listitem = list.Items[1];
listitem.Title = "Test List Item"
listitem.SystemUpdate();
list.Update();