"If you had just $500 to spend to upgrade the stock sound system (basic 6 disk changer) in a Titan and limited installation experience what components would you recommend?"
I would recommend saving some more $.
If you have to blow the $500 now, I would either get satellite radio or get an LOC, amp & sub. Maybe 11KV2 sub + Nine 2 amp from
www.edesignaudio.com