Friends and I have had this issue in various Manhattan apartments, where, because of various co-op or condo rules, the solutions were limited, including murdering the offending neighbor.
I'm assuming you have drywall and not plaster walls.
You should try the Electrical Outlet Seals first - which might entail cutting a bit of the existing drywall away.
Sound is going to travel via any hole it can find, so that's the first place to deal with.
The solution we all found for non-plaster walls - and (relatively) little expense - was
http://www.quietrock.com/
Ideally, it would be great to pull down the sheetrock you have now, put in regular insulation (and/or that expanding foam stuff) and then the Quiet Rock on BOTH sides of the wall, always in conjunction with sealing the outlets using the Quiet Rock Putty.
Putting two sheets on one side does not, apparently, give you double the soundproofing.
But putting Quiet Rock over the existing drywall, using at least 3 tubes of glue per sheet, along with sealing the outlets, and caulking any other holes (on both sides), should dramatically contain the sound.
Generally the expanding foam is done before the drywall is up - that way you be sure you get every single nook and cranny.
However, if it's done after, there would have to lots of holes made to ensure you get full coverage (at least two - mid-point and by the ceiling - between each stud).
I'm not sure that's recommended.
Also, the blown-in insulation will eventually settle so it's not a good option
A drywall guy could easily get this done in an weekend (even including taking down the existing drywall).
It's not as big as mess as you would think.
Then it's just the painting.
And, as was often the case, it can ease the homicidal thoughts about your neighbor.