Installation of additional system memory is generally the least expensive but most effective upgrade to improve the speed and performance of your computer.

Installation of a new hard disk drive as either a replacement for the original drive to increase storage space for more music, picture and movie files, for example, or installation of a second drive for the same purpose is also a relatively inexpensive upgrade to your computer system (note: most notebooks and netbooks generally only have space for one hard disk drive; however, desktops can usually accommodate 2 or more drives).  For data backup, an external hard disk drive, via USB, eSATA or LAN (local area network), is a very useful addition to your computer system, especially if you have important or sensitive data on your hard disk drive that would warrant backup (i.e., duplication) to a second drive.

Installation of a new power supply may be required if you wish to install multiple hard disk drives and optical drives, or if you want to install a high-power gaming graphics card.  Newer power supplies support PCI-E graphics cards and SATA devices.

With a media card reader built into your computer case, downloading pictures from multiple media cards and different card types is more convenient than working with the media in your digital camera connected to the computer via USB cable.

Although most computer cases are equipped with at least one cooling fan, installation of additional fans or replacement of the original fan with a larger, more efficient case cooling fan can significantly enhance the cooling of critical components inside your computer case, such as the motherboard, CPU and hard disk drive(s).  With improved cooling, the lifespan of these components usually can be increased significantly, especially the hard disk drive, which is one of the system components that can least tolerate elevated temperatures.

Installation of a new graphics card (or video card) or replacement of the original card with one that has more on-card memory not only can improve the gaming experience, but also may be necessary for some games.  If the computer is not used for playing games, then board-embedded graphics should be sufficient.

Installation of a second CD player or CD burner will allow you to copy CDs "on-the-fly;" that is, will allow you to copy data to a new disc without having to remove the original to insert the blank CD-R or CD-RW.

Installation of a second DVD player or DVD burner will allow you to copy DVDs "on-the-fly;" that is, will allow you to copy data to a new disc without having to remove the original to insert the blank DVD-R or DVD-RW.  Most DVD players also play CDs; however, CD/DVD player/burner combo drives can read and write both CDs and DVDs.

Installation of a Blu-ray drive will allow you to play Blu-ray movies on your computer, as well as store up to 10 times more data on Blu-ray media than is possible with the DVD format.  Most Blu-ray drives also read and write CDs and DVDs.

Although made obsolete by USB flash media and external hard disk drives, installation of a floppy disk drive or use of an external floppy drive will provide access to data previously stored on floppy disks.

Replacement of your older ball mouse, which will eliminate the need of a mouse pad and constant cleaning of the ball, or optical mouse with a laser mouse will improve accuracy and sensitivity.  A laser or optical mouse does not use a ball to sense motion; instead, a beam of light strikes the surface and senses movement of the mouse relative to the surface.  The laser mouse, however, uses a narrower, more focused beam of light than the optical mouse (which was the first replacement for the ball mouse) and, therefore, is currently the most accurate and sensitive mouse on the market.  Both laser and optical mice have no moving parts, are easier to clean, are easier to "keep" clean and can be used on any surface (i.e., a mouse pad is not required).

Replacing your wired mouse and/or keyboard with a wireless mouse and/or keyboard will give you the freedom of placing the mouse and/or keyboard anywhere you wish without the inconvenience of those annoying "tethers."  Wireless devices now work very well, and moving a mouse around without dragging (and sometimes snagging) the wire is very appealing, as well as placing the keyboard anywhere you wish, including your lap, if that is your desire.

An ergonomic (or natural) keyboard is a keyboard that is more or less split in the middle with the two halves angled outward to better fit the natural angle of your arms as they are moved in toward the keyboard.  This type of keyboard eliminates strains/pains in your wrists because the hands and fingers can remain in, more or less, neutral positions.

An all-in-one printer, copier, scanner, FAX Machine (MFC) is a convenient peripheral when and where a printer, copier, scanner and/or FAX machine are needed but space is limited.

An excellent printer upgrade would be the replacement of your inkjet printer with a laser printer.  Laser printers are much faster than inkjet, the ink on paper will not run when splashed with water and there are no ink cartridges to dry up when the printer is not in use.  In recent years, the prices of laser printers (monochrome and color) have dropped considerably, making them a much more reasonable/affordable upgrade than in previous years

Although made obsolete by USB flash media and external hard disk drives, installation of a Zip drive or use of an external Zip drive will provide access to data previously stored on Zip media.

If your motherboard supports faster CPU's (processors), installation of one usually can significantly increase the speed and performance of your computer system.