Case
Case first, shall we? The case is there not just as a box, but it can allow your system significant flexibility. Most computer cases these days are tower type cases with sizes ranging from the small Mini-Towers to Mid-Towers and Full-Towers. As they increase in size, the availability for expansion increases as well. If your case will not allow the addition of drives due to not having sufficient drive bays, then you are stuck with the choice of a new case or of only adding external devices. As a rule, external devices (hard drives, CD-ROMs, etc.) usually cost more and provide slower performance. Not only that, but changing your existing system to a new case is no easy task. A second consideration is the power supply. Modern systems are in need of power supplies in excess of 300 watts, with upgrading to near top of the line technology demanding 400 watt power! If you have an adequate or a fancy modern case like the glass-side aluminum ones that are out that you want to keep for a long time, when you upgrade the next time, you can change out your power supply if you need more juice. Some cases have built-in flash memory readers and/or USB ports in the front or on the side that provide greater accessibility for external device connections. The choice of a case used to be based primarily on size but is now influenced by design features such as what they are made of, their color, plexiglass sides, extra LED fans, cold cathode lighting and so forth. You can even obtain water-cooled cases if you want or need them. If you want to get some ideas or see some of the choices in cases and case additions, then go to check out the web site Xoxide or Pimprig. 

Motherboard
Next the main part of your computer is your motherboard. Motherboards often must be upgraded when a computer is modified with a faster, newer CPU. However, your current motherboard may support a faster processor than the one you currently have installed. You must be very careful here, since there are so many different configurations and motherboards with a variety of different chipsets. Be sure if you are upgrading to a Pentium 4 chip from an Athlon or vice-versa that you either buy or already have a motherboard that will support your CPU chip. You have to make sure you are buying the right motherboard for the job, and it's a good idea to purchase a motherboard that allows you to upgrade to a faster CPU in the future. Other things to consider are the front side bus speed of the board, how many slots are available for expansion, what type of CPU and RAM the motherboard supports, and what is integrated into the motherboard (sound, graphics, modem, LAN, etc.). More recently, motherboards have been offered with RAID controls built-in. Most new motherboards also support 4X/8X AGP graphics cards.  On the horizon, serial ATA drive support is likely to be common among new motherboards. 

Processor
The processor or CPU is a key component in determining your computers speed. Generally, premium prices reflect top-of-the-line speed and processing power. Intel and AMD continue to race for dominance. Their price war and competition is generally good for consumers, as prices fall on second, third, and fourth line CPUs. Intel is often preferred for servers and high-end applications and offers good support. AMD chips are generally less expensive and often provide the home computer user better graphics capability. AMD has lagged behind Intel P4 chips for a few months but they appear to have made strides with their new AMD 2600, 2700 and 2800 chips, which, as of this writing are out but not yet available to the upgrading consumer.

Hard Drive
Hard drives are the primary data and application storage component in your computer. As they are becoming huge in size, they are getting faster also. The faster they spin, the easier it will be for them to locate data on the drive that the CPU or software needs. Earlier version IDE hard drives spin at 5400 RPM with newer drives increasing that to 7200 RPMs. Logically, the larger the drive, the more significant this speed difference becomes. Application intensive work such as video editing relies on hard drives that are fast enough to keep up with the program without losing data. 

Sound and Graphics cards
Cutting edge sound cards and video graphics cards provide significant enhancement in your computing power. Newer sound cards, in addition to the everyday analog sound,  offer 6-channel, digital sound which attempts to emulate theatre surround-sound and some newer motherboards even have this built into them nowadays. Video graphics cards are moving to 8X AGP, which means graphics data gets processed faster but these are not readily available and are still quite expensive. So, unless you want the fastest and best graphics capability out there, moving up may not be top priority. What does seem most important is that graphics cards are building in more power and support including DVD, TV and video editing and increasing memory built right onto the board. Currently, there are basic 16 MB cards with most cards having a minimum of 32 MB of onboard RAM on up to 64 MB and 128 MBs. Can 256 MB or even 512 MB graphics cards be very far away?

Monitor
Your monitor or video screen is a significant part of your computer. Older monitors have an analog connection with LCD flat-panel monitors offering DVI (Digital Video Interface) connection. 

As far as other peripherals, be sure that there is compatibility between peripherals that you want to keep and use with your upgraded system and your new hardware. Besides that, the basics for upgrading consist of deciding what you want to achieve when you are done upgrading and what you need to do to get there. If you have more questions, you can always search the Internet for help, as there are many helpful sites out there. Good luck in your upgrade project!