what do you get with an arm upgrade?

If you are Lee Majors - a bionic arm!

If the arm is truly an upgrade, then you'll get better sound overall, with the appropriate matching cartridge. Exactly how it will sound depends on the table, arm, and cartridge used. Expect less colorations, better detail, possibly better bass or more liquid presentation, more dynamics, more "lifelike" sound......

Could be alot of things. Depends on the combo and the setup.