But you cannot have a band or orchestra in your room sitting there 24h a day ready to obey your every musicial command. Here where MIDI becomes useful. You can record and play whole arrangement of your music without any live musicians, anytime you want. You can experiment with different arrangements, orchestrations etc. This gives you a huge advantage of perfecting your music.
Lets get to it a little deeper.
The speed of processor doesn't matter for MIDI only, but it matters very much for audio recordings. The size doesn't matter as mch as the speed of cache for audio. ANY keyboard you like, but make sure it has 61 or more keys, you can go with 49, bt it would be very hard to do any real-time piano playing. MIDI cables and interface. Can be parallel, serial or USB port.