How To Record & Display External ID In Contact Summary?
You could do this with a database trigger.  If you are using external id for the existing members you could put a trigger on civicrm_contacts after insert that would populate external_id with the internal_id.  (assuming after you import the internal_id will be higher than any existing members.  If not, than just add some number to get it higher).

External ID does show up on the main contact screen on the right side above phone number.  If this isn't prominent enough you could use the civicrm_contact_get_displayname hook to append the id to the end of their display name.

Here is some info on that.
http://wiki.civicrm.org/confluence/display/CRMDOC/hook_civicrm_contact_get_displayname 

Also, you'll find a question or two on this in the old forum and in this stackexchange.

Note:  if you add a db trigger you'll need to add it through the trigger_info hook.
http://wiki.civicrm.org/confluence/display/CRMDOC/hook_civicrm_trigger_info