The MBox is quite popular among music enthusiasts. It has an optimum number of input and outputs. In most cases you will probably be using nothing more than a couple of mics and the main stereo outputs for use with monitors. This hardware and software bundle will include many great plug-ins and software synthesizers to accommodate your every whim.
In a semi-professional home studio the MBox will also suffice. Though if you have aspirations of moving your music into a professional studio for more finessing, you may want to get the 003. This unit has enough ins and outs to record an entire drum kit. Of course you will still be able to handle the majority of the overdubs as well.
The contemporary professional typically requires an HD system. These systems are much larger and more expensive and can be expanded to fit a studio's growing demands.
My studio operates a rather massive system. Our criteria is geared towards mixing and the additional processing power is imperative. This is what I use everyday:
Digidesign ProTools HD 8 Software
Digidesign ProTools HD6 Accel
(1) Apogee 16X A/D Converter w/XHD Card
(1) Apogee 16X D/A Converter w/XHD Card
Magma 7-Slot/64 bit Chassis
Magma 4-Slot/32 bit Chassis w/2x Universal Audio Cards
Quite massive, but when speed and efficiency are required we can't afford to cut corners.
Keep in mind that with the technological advancements in computer power the need for a full blown PTHD system lessens. Computers are now capable of providing more processing power than the most advanced systems. Most of us who still maintain large ProTools HD systems already have a vested interest in them. The cost and downtime required to migrate into a newer system is a frightening proposition.
So my advice if you are getting a system for the first time is to get a new computer. This way you can in many ways rival an HD studio in processing power. The ability to run multiple instances of plug-ins, soft-synths and high track count can be accomplished mush easier on a newer, more powerful machine.