Depends hoopla on the system you buy, most 5.1s come with a decent amount of wire length, doesn't matter though, worst case scenario you have to buy some extra hookup wire for the rear satellites.
500 dollars will get you an audio system that will piss off everyone in the entire building.
I'm split on Bose. They cost WAY too much, they are however high quality, I don't recommend them though just because the extra cost is not worth the actual quality improvement.
You could do far better with a 200-300 dollar spending limit on a home theater style surround system, 7.1 or 5.1 your choice, just make sure your PC can output surround audio to them. I have a PC about 3 years old that has a digital out that will do full 7.1 output over a single RCA connector. Many systems have optical in/outs as well (uses the same format as the wire but over optical fiber) so just make sure you have the actual system connection part straight before you buy an audio system that can only be fed a 2.1 signal =)
My father recently bought a DVD/blueray player combo that had a built in surround system, was only 300 dollars and he can never turn it up all the way or he'll tick his neighbors off. It's hard to go wrong with commercial surround systems nowdays.
I did try on a pair of Bose Quiet Comfort 15 noise reduction headphones at bestbuy the other day. Putting those things on is like having the world turned off, it's startlingly quiet and the audio quality seemed really good though it was just a demo so I didn't get to put any really taxing music through it. It's 499 for a pair of headphones though.... That's basically what you can expect from Bose, 50-75% better audio. 500% extra cost.