What can I expect in sound quality with an upgrade?
Is this a rethorical?
IMHO, it is more a question of, what are you already missing, than, what more can you get.
Go to high end AV stores and demo their stereo setups, price no object. Once you hear what you like, try to reproduce that on your own setup.
The speakers, their placement and room acoustics play a much much greater role in sound quality than electronics. The first step is to determine what you are missing? Try doing a FR plot to determine if you are getting the most from your setup. If there are any major peaks or dips, try correcting them. Get as close as possible to a flat FR in-room (I believe the best FR is supposed to have a slight slope from low to high frequencies).
Once you have the best possible sound from your own setup, return to the demo stereo at the showroom and compare again. Now you will be able to truly say if you are missing something or not.
Needless to say, use the same media and make sure it is a good quality recording when doing subjective listening.