SDRAM

SDRAM comes in three speeds; 66MHz, 100Mhz and 133Mhz. If your computer is about 4 years old, chances are you have 100Mhz or 133Mhz installed. However if the computer is about 10 years old then you are probably running 66Mhz at best. If this is the case its probably best to purchase a whole new computer and start again as it will probably be easier and cheaper.

DDR RAM
DDR has a larger range of speeds than SDRAM and comes in 266Mhz (also known as PC2100), 333Mhz (PC2700), 400Mhz (PC3200) and 533Mhz (PC4000).

DDR2 RAM
DDR2 is the second generation of DDR and is faster than its predecessor. However, DDR2 only works with certain Intel Pentium 4 chipsets (Althon64 doesnt currently support it). Correction: AMD64 does now support DDR2.

Starting the upgrade
Now that you are armed with this information you need to find out what type of RAM is currently installed in your computer. You could open up your computers case and physically look, However you need to know what you are looking for. Another option is to download a handy piece of free software called Belarc Advisor which will tell you what type of memory you currently have installed and wether you have any free physical slots available.