Mac/PC - Network them together?

Hey all, sorry for the dumb question. Im hoping its a simple solution.

I have 3 PCs and 1 Macbook at my house. One PC is my sons computer for games, etc... Another is my work computer (home business), and last PC was my personal computer for photos, videos, etc.. until I bought the Macbook recently.

So I basically have 4 computers at the house. I have ATT DSL and a wireless router hooked up to my original PC. I then connect the other 3 computers to internet via wirelessly...

Here is my question. I was given a 320GB external hard drive to me for free from a friend. I want to try and sort through all my photos, videos, files and store them on this drive.

Is there a way I can easily network all my computers together so I can share files between them all? As well as this external hard drive?

Also, is there a way when Im away from the house I can log into my network and download a file or something?

Thansk and again, sorry for my ignorance.