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" drywall, with a coating of Green Glue in-between. 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 diagram from Trademark Soundproofing that demonstrates the STC results between different wall assemblies, including those with Green Glue.
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 when you add QuietRock directly onto the wood framing, you're still not decoupling it from the structure. And therefore, impact noise and lower frequencies might 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 might not resonate through the layer of Green Glue to the layer of new drywall -- at least not to my ears. And any gaps in coverage of the Green Glue between the two drywall layers will also be mitigated by the layer of dampening material that's already inside the QuietRock drywall.
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."