Let's see
M-Audio Keystation 88 ES 88-Key MIDI Controller
With this keyboard you will get full 88 key keyboard with velocity sensitivity represents piano keyboard. 200$
The disadvantage of this keyboard that it has very few control knobs and dials. Not a broblem for me, but many peopla like to have a lot of control posibility.
M-Audio Keystation 61es 61-Key MIDI Controller
A younger brother (or sister maybe) of the above one. 61 keys, the rest is the same. 170$
Behringer UMX61 61-Key MIDI Controller
"Cheap and angry" solution from Behringer. More controls, 61 key, 50 virtual instruments included.
Emu Xboard 49 49-Key MIDI Controller
Well yes 49 keys only, but with aftertouch function and enough of controls. Comes with proteus software that includes 1000 of sounds and virtual instruments. I vote for this one! 190$
Now spme technical stuff.
1. USB will not give you any problems with firewire. It will work.
2. Velosity sensitive means the keyboard will react on how "hard" you press the keys, it will follow dynamics of your performance.
3. Aftretouch
Most professional and more and more simple keyboards come with aftertouch these days. Aftertouch can be used in different ways. The most basic and standard use for it is to change the volume of a sound when you are already playing it. Lets asume you are playing a note on your keyboard. To simulate a real piano the note doesn't have to get louder. You would hit the note of chord and it would play and slowly fade away to silence. But what if you would want to play a saxophone or a string on your keyboard or synthesizer? These instruments have the ability to start a note and get louder after that.
To simulate this aftertouch was invented. When you play the note on the keyboard the sound starts. The keyboard or synthesizer has a built in system under each key which measures the pressure. So when you have played the note you can make the sound louder by pressing harder, hence the name aftertouch. So this gives you the possiblity as a keyboard player to simulate strings, woodwinds and brass instruments and sustain guitar notes longer.
Since the introduction of synthesizers aftertouch is used for many other things too. For instance it can be used to control the modulation of a note instead of using the modulation wheel, or for controlling the tremolo, or changing a note into a different sound.
Cheerz! Good luck in choosing the right stuff!