Different Types of OS
As you know, there are many different Operating Systems, or 'OS's', available today. These include Windows and Linux. Windows, made by Microsoft, is many peoples choice as most people are familiar with it. Linux is for people who like to manually tweak and alter their computer, although not exclusively limited to this due to the arrival of Desktop environments such as "Gnome" or "KDE".
OS Requirements
Operating Systems vary in their resource usage. OS's like Windows 98, for example, require much less resources than the current Windows 7. This is due to technological advances which allow software developers to make their products "better". Linux generally requires less resources than Windows. For current versions of OS's, Windows runs comfortably on 2GB of RAM or more, and Linux runs comfortably on 512MB or more, depending on usage.
Which Should I Use?
This question is asked every day, but there is no simple answer. Depending on what you want to be able to do, you should decide on an operating system before building your PC.
As a piece of general advice, if you are asking this question the hardcore versions of linux really aren't for you. Generally a Microsoft opperating system will probably be what you are used to.
If you have your heart set on linux and are asking this why not try Ubuntu. Its easy to use and set up (its also compleetly free!)
32 or 64 bit?
There is one major distinction you will need to know about when installing an OS and that is whether or not to go for the 32 or 64 bit versions of an OS.
What's the difference you ask?
A 32 bit OS can only really address about 4GB of ram (it can technically use more but its tricky and less efficient)
A 64 bi OS can address 192GB of ram
Chances are, you will never buy more than 4GB of ram. If you do, you probably know what kind of OS you need.