1) From where are you getting your TV signal - from antenna, cable box, satellite receiver????
2) Yes, you will probably need at least a couple of additional cables to connect your devices to the EyeTV. You will need to look at your devices, the TV, cable box, satellite receiver, etc. and see what type of connections are available on them for output.
3) The only content you will be able to record in HD will be by using the NTSC tuner (aka digital tuner or HD tuner) in the EyeTV to get your local over-the-air channels that are picked up by an external antenna and the antenna is connected to the EyeTV with a coax cable.
4) There are three ways to get a signal into the EyeTV.
A) Coax cable connected to an external antenna to receive your over-the-air local TV channels - like ABC, NBC, CBS, Fox, if you don't know what channels are your locals vs those fed from a cable or satellite box - ????
B) S-Video cable - this is the one you will use to connect your Xbox and most other equipment - it will not be in HD when it makes it into your computer as this cable does not have the capability of passing through HD quality signals - it should be as good as DVD quality
C) Composite cable - you do not want to use this connection except as a last resort as it will have the worst signal of the lot - definitely not in HD
Of these three cables, only the coax will also provide you with the audio portion of the content. With 'B' or 'C' you will also need to connect audio cables to get the sound from the device.
For me to be any more specific - you will have to give me your specific TV model, cable box or satllite model, etc.
5) Unless you are just watching a show and/or burning it to DVD and then deletling it - you will very rapidly fill up your hard drive. If you are wanting to save your recorded stuff, you will need more external storage space, sooner rather than later.