Open Source and Dual Licensing

Help me parse through the legal mumbo-jumbo.

Basically, what I’m doing now is developing an open source Flash project, which I will release. What I want to do is license it in a way that the end user is free to modify it, as long as the source code to the modification is released. What I don’t want to see happen is the end user downloading the project, packaging it and selling it to companies for profit, although I would like to retain the right to do this (license my copyrighted work). As far as I know the GPL would cover this, the only thing I’m not 100% on is licensing the code I own the copyright to. As the copyright owner, I’m quite sure I would have this power, seeing as how I would theoretically be licensing the software on a lesser license, which I have the power to do.

Am I completely off here?