Upgrading the memory on your Mac is the cheapest upgrade you can make but it will make a difference by boosting the amount of memory your system has you can run more tasks and programs at a faster rate. If you have ever upgraded the memory on a Computer then let me reassure you that it is essentailly the same process for upgrading a Mac.
USA Link Memory upgrades for your Apple Mac computer
UK Link Crucial memory upgrades for your Apple Mac computer
What you need to do is find out the type of Ram/Memory your system uses, this will normally be DDR, DDR2, PC133. The easiest way is to either read the specification on the vendors website or manual, or if you are happy to top open your Mac and find the exisisting memory makign a note of the type mentioned on the sticker on the memory or take out the memory and take it to your local store. Remebering to take care not to touch the edges or let static etc come into contact with the memory.
For more information on PC memory which is the same type used in your Mac computer click here: Link