how much storage needed? or how do I find out?

I've been using Macs for years but feel as though I don't really know them well.

I'm upgrading my son's MacBook (white, mid-2009) to Snow Leopard and before I do so I want to back up the computer. I have a 500 gb external hard drive available (it isn't full but I'm not sure w

My questions are:

1) how can you find out how much space you'll need for a backup?

2) what is the easiest and/or fastest way to perform the backup?

Thanks!