http://blogs.abc.net.au/articulate/2009/09/movie-minutiae-ghostbusters-1984.html

It's kind of hard to believe this flick is now 25 years old.

Initially slated to feature John Belushi, Dan Akroyd, Eddie Murphy and John Candy, things went pear-shaped following Belushi's death and the script was re-written with new actors cast.

Original script-writer Akroyd remained on board and was joined by fellow writer Harold Ramis and a then 34-year-old Bill Murray.

Christopher Walken, John Lithgow, Christopher Lloyd and Jeff Goldblum were considered for Ramis' role, but he opted to take it on himself because he felt he was best suited.
Rick Moranis took the part intended for Candy.

Murphy instead ended up doing Beverly Hills Cop, which beat out Ghostbusters for the highest-grossing film that year.

Ray Parker Jnr apparently came up with the idea for the catchy theme tune at 4:30 one morning when he saw a TV ad for a drain company.

The spark helped coin "Who you gonna call?"

Parker had been scrambling to meet his deadline and was forced to
use his girlfriend and her mates for the chorus shouting "Ghostbusters!"
The song turned out to be a number one hit for three weeks in the US.
Parker was later sued by Huey Lewis and the News, who claimed the tune was plagiarised from their song "I Want a New Drug."

You can compare the two below (they do sound remarkably similar...)

'Ghostbusters theme'

'I Want a New Drug'

Ghostbusters spawned a sequel five years later and Ramis confirmed last year that a third film is in the works, with Judd Apatow attached and the original actors involved in some capacity.

wow the songs do sound pretty simalar, but i like Rays better lol