So what part of the States are you located?
You can run a 520 chain with no issues on this bike.
As for what the numbers mean...520,525,530 and so on here is a good reference:
For gearing, personally I would either go to a 43 on the rear (preferred) or a 15 on the front (it works but is a pretty tight bend for the chain)
Some folks go lower but that is a discussion for another time....lol
Also keep in mind that when you change the sprockets you will also change the wheel base of the bike.
43 out back will make it shorter and a 15 in front will make it longer.

Loud pipes don't save lives, knowing how to ride your bike will save your life.