I'm also interested in making my car more of an aggressive look, I'm not sure I would like wheels jutting out of the fenders though,
Honestly, in my opinion I just think the stock rims don't look aggressive at all and would need a decent set of 18's to start. Then a louder exhaust to untame the beast
You might want to check this thread and just look at pictures of other cars before you decide on what you want to do.
Post Pics. Boxsters with Rims.