If you are just going to surf the net and write emails, then space is not really a factor. Keep in mind that you want one with 3D accelerator support and at least 8 MB of memory on the card. Anything less will be a very bad performer. Also consider what you will be using your PC for. If you are just surfing the internet and writing email, then smaller and cheaper is probably better.
Most modems are fairly standard.
If you intend to connect to the internet via phone then get a 56k modem. If you plan to connect via DSL or Cable, then you will need a Network Card. They are all about the same, so don't worry too much about them. A lot of major PC manufacturers make good machines, but they also integrate much of the hardware for your system in order to cut prices. They will also use parts that are not warranted by the manufacturer.