Installing OS X on Dell Inspiron 1545 currently on Vista (32bit)

I want to change my OS to a Mac OS X!
Im currently on Vista Home Basic SP2.
As stated i have a:
Dell Inspiron 1545 Laptop
Intel Pentium Dual CPU T3400
Processor Speed: 2.16Gz
RAM: 2GB
Hard Drive: 140GB
they're all of the specs i know, if you need to know anymore you'll have to tell me how i would find out too!

Most people seem to be switching to OS Snow Leopard, but would Leopard be an easier installation? i don't mind!

In summary, i hate to be a pain in the arse but im completely new to all this new operating system installation and OSx86, so i need someone to help me with a complete guide on how a approach this from start to finish. to illustrate, what sort of things other than my personal files do i need to back up (like programs and stuff?) and do i just bung all of these on an external hard drive?

Also is there a way i can just return back to my original OS, if installation screws up for any reason? i dont have any of my old disks...

Granted this may take quite a while as i only have a vague understanding, and when telling me what i need to do, please explain everything in detail otherwise i doubt i'll understand, but help would be greatly appreciated!

Thanks Guys!