Generally speaking, computer memory upgrades (not including Flash memory cards or USB drives) can be divided into two different types:

Type 1: Generic upgrades. These are made by a plethora of manufacturers and are made to a speed and form factor specification, such as a DDR-333 200 pin SO-DIMM, or a DDR2-667 240 pin DIMM. Generic upgrades are made without regard to the particular requirements of the computer they are being installed in. Generic upgrades are usually cheaper and supplied by 1000's of resellers worldwide.

Type 2: System specific upgrades. (Also known as 'branded' memory). This is memory that is made to a particular speed and form factor specification, and further tested to function in particular brand / model computers and motherboards. The most well known manufacturer of system specific upgrades is Kingston.

In recent years other memory manufacturers have also chosen to compete in the branded memory market, such as Transcend, Corsair, A-data, and Patriot. All offer similar guarantees of compatibility, however Kingston is arguably the most trusted brand, especially amongst those who work in the IT sector. Kingston also have close relationships with the major OEM's like IBM, such that technicians will service onsite Kingston memory products that have been installed in IBM computer systems.

If you purchase a system specific module(s) for your computer, Kingston will guarantee it to be compatible. System specific upgrades are generally a little more expensive than generic upgrades because the design is customized to meet the memory requirements of the computer system the module is designed for. Kingston also also tests the module in the end system using popular applications that people use every day (email clients, web browsers, etc), operating systems and the original equipment manufacturer's own system diagnostic program.

So, let's say you own a Dell Latitude D830 model laptop and you want to upgrade the memory. You have referred to your owners manual and you've figured out that your computer takes DDR2-667 (also known as PC2-5300) 200-pin SO-DIMMs. You could now just find the retailer with the cheapest DDR2-667 SO-DIMMs on the internet and duly purchase them. I see this as a lottery. Chances are you may be lucky and the modules may work. However, like many people who buy the cheapest memory they can find, and then end up purchasing the correct upgrade from RamCity, you may have problems with modules made using a lower quality manufacturing process. In many cases these people lost their money because their purchase came without any warranty as to compatibility and the reseller argued that their purchase was not faulty, just incompatible.

So, if you are in the market for a RAM upgrade, and want some peace of mind and reassurance that it is going to work the first time, be sure to ask your reseller if they sell system specific memory upgrades. Also check if they offer a guarantee that their modules will work in your computer, or happily refund your money in full.