I lol'd.
If you want aggressive, paint shark teeth on the front and put a fin on top. =P
No really though, you'll get aggressive from a Ram Air hood with raised scoops, F1 hood, SD Hood, MPD hood, WS6 Hood (WS6 hood looks gay though).
You'll also get it from a big lip spoiler. There are a couple big lip spoilers out there, some stick OUT more, some stick UP more. I personally like that ones that are up more, because I prefer not have have my spoiler hanging 6 inches off the end of my car.
Tinted windows, and a well-played color scheme also add to aggressiveness. Like black on a red car, gunmetal on a black or maroon car, etc.