I'll assume you're referring to power modifications. Before you spend any money on your car, you should decide exactly what you want out of your car. Is it your daily driver? Are there emission standards in your area that you need to adhere to? Exactly how much money are you willing to drop into it?
Well, aesthetics are obviously a matter of personal taste. If you want to keep it nice and simple...a clean set of wheels, HID's, and a tint should do it, IMO. But as stated, that's simply my opinion. Do what you think looks nice. How you feel about your car is what's most important.
As for handling...Sway bars, springs and shocks (coilovers if you feel like dropping a bit more), tie bars, etc.
Springs, shocks, rear sway-bar, and tires are what you should prioritize, IMO.
For springs, I wouldn't get anything lower than a 1.5" drop if you're concern is performance, rather than looks. I have Eibach Pro-Kits and I dig em. I also hear good things about the Swift springs.
For shocks, you can't go wrong with Koni Yellow's. If you're looking for higher performance OE dampers, Tokico is good stuff.
For the sway bar, Progress is the most popular. Eibach is another decent choice.
Tires are a matter of personal preference, depending on your local weather conditions.