You can add mass to your existing walls.As I mentioned in a previous entry, I successfully muted some neighbor noise in my apartment building when I added a second layer of drywall on top of the original 1/2" thick wall, with a coating of Green Glue inbetween. This produced better results than simply replacing the original wall with QuietRock 525. I made the double-drywall solution even more successful by using QuietRock 510 as the second layer of drywall, with Green Glue in between the two.
Here's a chart from the Green Glue Company that demonstrates the STC results between different wall solutions. Granted, it's on their web site and not entirely objective -- but because I've had good results myself, I think it's worth noting.
Why do I think the Green Glue + layer of QuietRock was better than just replacing the original wall with QuietRock, right on the studs? My theory is that by adding QuietRock directly onto the wood framing, you're still not decoupling it from the structure, and therefore, impact noise will still come through even though conversation and other frequencies of noise might be reduced. However, by adding a second layer of drywall on top of a viscous layer of Green Glue, some of the impact noise from the framing never resonates through the layer of Green Glue to the layer of new drywall -- at least not to my ears.
Of course, sound travels in many different ways -- not just through the walls. Here's a quick primer from Trademark Soundproofing in the "5 Principles of Soundproofing."