How to wipe hardrive clean?

Hey all, have a few questions. I have an old macbook, the 10.5 version and want to sell it. The problem is that it contains important things I don't want to be recovered so I want to wipe the hard drive. My question is how should I sell it the cheapest? I don't have the old installation disk, and those things are like 150 dollars. Would it be better to upgrade to snow leopard, wipe, and then sell it?

My other question is if I went on ebay and got a disk, would it need a serial number or any other junk or could I just wipe the hard drive clean with it? Thanks.

OSX does not require a serial number

But you could sell it as is

Get all your stuff off and create a new admin account (the details of which you will give to the new owner)

Delete your account from the computer leaving the new admin account (make sure you create and delete accounts from the Accounts pref pane) and delete any applications you dont want on there with app cleaner (google that app). Empty the trash

Boot her up to the new admin account and open disk utility, select the erase tab and select Erase free space

If you are extra paranoid select a few large files on a usb stick (like a 2 gig movie and copy it to the hard drive. Duplicate that file untill you only have a few gigs left then delete the files and run erase free space again.

Now you have a hard drive without your account and info that has had files written over and trashed so that there is no chance that anybody could recover the old stuff