6Gb doesn't give you much room to play with but you should be able to squeeze most distros on that.
Basics of Linux.
As you already know it's an Operating System. Unlike Windows there are MANY, MANY versions or "distributions" (distros) available and you could spend months, even years finding "the one" that suits you. There are also many desktop environments available for each distribution for example Gnome, KDE, XFCE, Enlightenment, Fluxbox etc, not only do they change the appearance of the Desktop, but also how it operates and of course you can customise each one to your liking. A fair few distros offer a Desktop or Live CD that will run from the CD to give you a taster and to allow you to test which hardware works.
Software - You should be able to find a Linux alternative to all the windows apps you have previously used and the good part is that they will be free! Some apps you have used may have a Linux version for example Firefox and GoogleEarth both have native Linux versions. Depending on the distro you are trying will depend on what software is installed by default. Some give you the bare minimum and some try to give you what they think you will need such as an Office Suit, CD burning apps, FTP apps, a choice of web browser etc.
You've probably heard that Linux is very secure, please remember that if you are not behind a router with NAT that you still need firewall software. antivirus is not necessary, and although you will find antivirus software for Linux it's really meant for running on servers to keep windows boxes safe. If you're on a network and worried about your windows machines then the av on the windows machines should be all that's needed, however Linux is all about freedom so go ahead and install AV on your Linux box if you want.
Gaming - It's hit and miss, depends if you are a big gamer or not and what you are into. Unreal Tournament 2004, Doom III, Quake 4 will run under Linux as they have native installers. Other "windows" games require that you run through a 3rd party app such as WINE or Cedega, but then only SOME games will work. Personally I have Civilisation IV running just fine through Cedega. There are of course free games available, however don't expect too much from them. If you're into your emulation and roms then there are also emulators that run under Linux, I have a Sega Megadrive emulator
Benefits of Linux:
In most cases it's free.
Free updates & upgrades.
Release cycle is very short with some being 6 months or less.
Access to thousands of free applications.
Extremely customisable both visually and under the hood.
Spyware has no affect.
Malware has no affect.
No Viruses.
Can bring old machines back to life that struggle with Windows.
Many desktop environments to choose from with varying footprints.
You can have effects much better than Vista's Aero without having to have Vista specs.
Disadvantages:
Very few windows games work.
Very steep learning curve.
No all hardware will work 100%, however different distros may have better support for it.
Having to use alternatives to windows software means you will also have to learn how to use that.
There's probably a lot more to be said and I'm sure other people will post.
I'm not sure how old the parts you are sticking together are but when I first tried Linux I used it on a very old machine with built in graphics and had problems, when I tried it on my main pc it worked "out of the box".
