The one in bold type is the component with the longest projected useful lifespan. It is also usually the costliest. If you stick to the bold recommendations, your system should run future mainstream applications without upgrading for around 4 years. Consider an ergonomic keyboard if you do a lot of typing. Sound Card ISA or PCI General MIDI compatible.
Digital outputs are nice when matched with digital speakers or digital stereo equipment.
Speakers Listen before you buy if possible. 5.1 systems are nice for home theater setups. Speakers built in to the monitor are great for reducing desk clutter. If computer will be using the Ethernet for connecting. If computer will be using phone lines for connecting.