Help me find an upgrade path for my '99 Cobra

So I have a '99 Mustang Cobra, with some basic bolt-ons.

The car is paid off, and is in great shape. There is nothing wrong with the car, and it has about 72k original miles.

I'm trying to decide what the best upgrade path would be for this car, or if I should simply sell it, and get something else. If I get something else, it would be either: a Fox Body GT, '03-04 Cobra, or a 2005+ GT.

At this point I'm probably looking at serious engine and transmission work to handle additional power on my 99 Cobra. I don't even know where to begin, or what my engine options are.

I'd like a high horsepower street/strip machine. Something that can be turned into a project car, and phased out of a daily driver.

Here are the mods:
Exhaust:

BBK Chrome long-tube headers
BBK X-pipe with high-flow catalytic converters
Magnaflow 2.5" stainless steel catback system


Intake:
Cold air intake with K&N air filter
(I do not know if the intake manifold has been ported)

Drivetrain:

3.73 Ford rear-end gears
Centerforce Dual-Friction clutch kit
Short-throw shift kit



Other Performance Mods:

SCT Xcalibrator 3 programmer
Underdrive pulleys

Here are some pics, just FYI:
http://i48.tinypic.com/11ue1jk.jpg
http://i46.tinypic.com/30xcwlg.jpg
http://i47.tinypic.com/2dr6w54.jpg
http://i50.tinypic.com/xkwt8i.jpg
http://i49.tinypic.com/nlo9p0.jpg
http://i45.tinypic.com/143dyt4.jpg



Give me an overview big-picture kinda thing of where to go from here? (and what might be involved)

I guess I should define my goals. I'm not interested in a pure drag car.

I would like something with plenty of power, but that is more like a road racing car. I want stiff chassis, good handling, brakes, and excellent power throughout the power band.