Asking what "the best <insert product here>" is a matter of preference and typically people will suggest the high priced items, regardless whether they use it or not themselves.
A preamp isn't needed once you have a mixer right.
Depends on the mixer, some mixers have built in preamps, some don't. If your board doesn't have a preamp, then you need to get one/a few. If it does have built in preamps, then you can either, A) Be happy with them, or B) try others and not use the ones on the board.
How do the headphones work/ what is the major funtion of them. Cant you run them into the recording booth so the singer can hear the music and sing along?
Headphones are headphones are headphones, same concept as anything else that has a headphone jack. You plug headphones in and listen. You can be anywhere you want, as long as you've got the cable length (or wireless headphones /puke) You can also utilize a headphone amp that you send a signal to, that will increase the volume and split the signal so you can have multiple people with headphones listening to the same feed at the same time.
What is the difference in a PCI, USB, and Firewire interface.
PCI slots are accesible on desktop computers, PCI cards are mounted within the PC case and the ins and outs on a PCI soundcard are on the back of the card, or a breakout cable that is attached to the back of the card.
Firewire and USB are other types of connection interfaces, these are compatable with any PC/laptop with a firewire and/or USB connections. These units are basically a "stand alone" box with a firewire or USB cable running to the PC or laptop. As to which is the best, I would suggest reading up on all of them and figuring out what is going to suit your needs the best.
Can you plug a MPC16 beat pad into a mixer on a computer based set-up. How exactly does this machine work do you use sounds on your sequencer or upload sounds......?
Can you provide more info on this?