It's especially important because, while the type of memory -- whether it be DDR, DDR2, or some other -- is locked in stone by which motherboard and processor your system works with, you get to choose the amount of memory that comes with a new machine (and add to it later).
However, it's not easy to figure out how much is enough -- computer memory is situational. What you're doing and the software you're using to do it are the deciding factors in determining the optimal memory size for your computer -- and they can change from PC to PC.
For example, according to Microsoft, all you need to run the Professional version of its Windows XP operating system is "128 megabytes (MB) of RAM or higher recommended (64 MB minimum supported; may limit performance and some features)." There's a minimum specification for the processor as well, but let's face it, chances are that your processor is well beyond that minimum.
In other words, my ancient and puny IBM ThinkPad 600X with its 64MB of memory should run Windows XP Pro. Stop laughing. It can -- to a point. Microsoft Word and Lotus Notes sail along smoothly. But that's about as far as you'll get. Windows is crafty -- instead of just grinding down to a screeching halt if it looks like memory's coming up short, Windows starts to use your hard disk as if it were memory, polling data to and from the drive as needed. The difference in speed (and, therefore, overall performance) is like that between walking and driving a NASCAR racecar.