A power amp is for powering speakers............you can figure out the rest. You will need some closed back headphones with good isolation (sound leakage) for the drummer to use while recording drums. The monitors, after being powered, typically plug into the outputs of the sound card. They can also be routed to the outs of a mixer.
Ok. When you say interface, I don't think you really understand exactly what an interface is. Or, maybe you do and I just feel like spouting. One of those all in one boxes, like the ones that you left links for in your earlier post, is really like having a preamp, sound card, and D/A converter all in one. Here is a simplified breakdown of a signal chain:
source (such as a guitar amp or voice) >
room>
microphone >
preamp (such as a mic preamp/direct box or a mixer) >
analog to digital converter (stand alone adc or part of a sound card or all in one interface>
digital info processed by the sound card or all in one interface>
software >
digital info processed by the soundcard or all in one interface>
digital to analog converter (stand alone dac or part of a sound card or all in one interface>
power amplifier for speakers>
monitor speakers>
room>
or a more simplified list of the pieces of the signal chain are:
source
room
microphone
preamp
adc
sound card
software
dac
power amp
speakers
room
All of these factors are important in getting the best sound for your money (or so I hear

). There are units that contain the preamp, converters, and dsp processing like the ones you listed. You can also get a sound card that includes the converters and dsp processing and then get seperate outboard preamps. Or, you could get seperate preamps, converters, and sound card (big bucks). For condensor mics, which you already know, you need phantom power. Phantom power can be included on an all in one interface, a mixer, a mic preamp, or a seperate unit. Headphone outs usually are'nt included on the interface. What is included are outputs that can be sent to a headphone amp or mixer.
I think the best bang for your buck is gonna be to get a soundcard with alot of inputs/outputs, and a decent mixer. A mixer will give you preamps, signal routing options, a headphone output, and phantom power. I have a Yamaha MG 12/4 mixer and I hear that it's good for the price, which it probably is. I can tell you that it certainly does'nt sound wonderful but I also hear that the Behringer mixers sound worse. I have'nt tried a Behringer mixer so take that for what it's worth.
What kind of sound quality are you trying to achieve and what is your budget? I don't want to discourage you but an interface and software are only a small piece of the puzzle in getting a good quality sound. You have to have realistic expectations of what you will be able to do within your budget. You also have to factor all the other major and minor pieces into your budget: good quiet recording space, room treatment, microphones, monitor speakers, headphones, computer upgrades, outboard gear, cables, stands, etc. You also have to try and balance the quality of all your gear to get the best bang for your buck. It makes no sense to put a really high quality piece of gear in an other wise shitty signal chain. It's just a waste of money. But, buying a bunch of cheap ass gear and expecting studio quality is also a waste.
After you get all the gear needed, you will need skills in mic placement, mixing, eqing, compressing, effects processing, etc. All of these things are every bit as important as the gear that you purchase. I have heard songs on this board that were recorded with minimal cheap gear that made me say "wow!, you recorded that song with that gear?" I have then used better or equal gear and not been able to touch that kind of quality recording. On the other hand, I have heard studio recordings that made me cringe.
There are alot of things to think about and research and this is a very good place to start doing that. I would start talking to alot of people around here and ask alot of questions. Go to the MP3 mixing thread and listen to some recordings that others have done and start asking what their recording setups consist of and what kind of experience they have in recording. I don't know what your experience is but there are alot of learning curves to get thru.
Also, you will need to find out which people here are genuinely helpful, which ones are bullshitters, which ones are company reps, and which ones are gear snobs. There are also some people around here that are very helpful but are also a bit rude to newbs so get used to that and don't let it bother you. Put some thought into your questions before asking, be specific, and be polite. Most of your questions can be answered by using the search function of this bbs so get familiar with it. One more piece of advice: the microphone thread can get real shitty at times; stay out of that scene unless you really need an answer to a question. One more and I'm outa here. Don't get GLS = gear lust syndrome. You will never be happy and you won't get shit done.
Good luck and welcome to the board.