If a sound card is included on your motherboard, it may be enough for your needs. You can add your own sound card if you're very concerned with audio quality (for things like gaming).
Today's hard drives typically use a SATA interface to connect to the motherboard. This interface uses cables a lot more convenient than IDE cables. You should probably be thinking a minimum of 500GB here, if you plan to use the system for several years.
Solid State Drives (SSD) are much faster than the traditional hard drive and typically use less power.
Speakers really depend on your needs. A cheap pair may work, an expensive pair may work. Speakers typically use a stereo headphone mini-jack (3.5mm) or USB. (Optical audio connections may be available depending on your requirements.)
Your LCD purchase depends largely on your exact needs. Some people may be fine with a 17 inch LCD with 1280x1024 pixels. Other users may prefer a High-Def system at 1920x1080, or something in-between. Check your video card abilities and make sure it can fully use your LCD.
LCDs are typically connected with an HDMI cable or a cable called DVI (which is essentially HDMI that doesn't carry audio), although VGA can still be used on many video cards. If your video adapter has a DVI port, you can often get a cable to use it with a VGA-ported LCD.
Keyboards are typically USB, proprietary wireless, or Bluetooth. You can get a standard keyboard or one with extra buttons, sometimes customizable.
Mice are typically USB, proprietary wireless, or Bluetooth. I'd highly recommend getting an optical mouse regardless of your connection type.
Your system will probably want an optical burner of some sort. Most systems will be fine with a DVD-RW drive, which will play DVDs, CDs, and burn DVDs and CDs. You might want to add a Blu-ray drive if you're using an LCD that's high-def.
I'd suggest you plug your system into a surge protector or uninterruptible power supply. The surge protector will also allow you to hook up your systems power cords to one outlet. Adding UPS can be a great safety feature if you don't want your computer to crash when the power goes out. It will allow you to have a few minutes to save your spreadsheet (or game).
Ethernet cards are typically rated at speeds of 10/100 mbps or 10/100/1000 mbps. Your motherboard may include one. If you're solely relying of Wi-Fi wireless for your network, you don't need to worry about this part.
You'll need a wireless network card (typically USB) if you're using your computer with a wireless network. Make sure the format is compatible with your access point or router. Most people use 802.11g or 802.11n networks, although 802.11b and 802.11a are also available. This can be an interesting thing to research (I find the standards fascinating).
If your motherboard doesn't include USB, you can add a card to it. If you want extra ports beyond what your motherboard includes, you can add one, too.
Some people use Windows, which you'd have to buy a copy of to install on your system. If you're using Linux, you should be able to download the software for free. Ubuntu is a good choice.
If you're planning to use dial-up instead of high-speed broadband, you'll need a modem to connect to your ISP through your phone line.
I'm not going into detail on these last three items, because I don't have any particular recommendations.