There are a lot of packages out there ... varying from the very basic ('Donde es el baño?' ... 'Dode esta la cerveza?' ... 'Donde estan las chicas?') to the very comprehensive. And, yeah, you can get along with next-to-no Spanish, but the more you've got, the better you can get along.
I found a package called Penton's Spanish 24 that has been a big help to me - it's very comprehensive, with 24 CDs and seven booklets. Actually, it includes three courses - 'Learn In Your Car: Spanish,' nine CDs that start out very basic and build up into a pretty thorough treatment of grammar and verb tenses; 'VocabuLearn,' twelve CDs that help you build your vocabulary and memorize hundreds of typical phrases; and 'Immersion Plus Spanish,' three CDs of conversations in Spanish to help you get used to the language. I got a lot of good out of it.
The bottom line is - learn what you can before you go. Get at least a basic package, and work your way through it - play them in your car on the commute instead of The Morning Mouth on your local radio station - dig in with them. (And Casper is absolutely right about listening to Spanish music - and Spanish radio when you get more advanced!) Then, use it when you get there - the most advanced lessons will be just talking with the locals, especially the chicas!