First, you do not need to uninstall the operating system. You can re-install on top of it (archive & install on pre-10.6).
2008? It probably didn't come with Tiger then (the forum in which you posted). It probably came with Leopard. Do you really want to restore to Leopard? I would go with Snow Leopard at least.
1) Make sure your computer meets Snow Leopard technical specifications - http://support.apple.com/kb/SP575 This mostly means making sure it is an Intel Mac and has at least 1GB RAM (more is recommended).
2) Apple no longer carries 10.6 in its stores or online. Telephone Apple to purchase a Snow Leopard disc. In the USA: (800) 676-2775 (contact information for other countries: http://support.apple.com/kb/HE57)
Alternatively you can buy from online retailers but expect to pay much more than the original price which is the price still charged by Apple
3) Backing up your computer before upgrading is always a good idea.
4) After installing from the DVD, update your Snow Leopard installation using Software Update or manually downloading and installing the Mac OS X 10.6.8 Update Combo v.1.1 - http://support.apple.com/kb/DL1399
Your cheapest and simplest option is to ring Apple and purchase Snow Leopard. They may only make a nominal charge for lost original disks but even full cost is only about £20.
There are no other methods of doing a clean install.
You could spend hours trying to remove private data without destroying the OS and still fail to remove something critical.
Strictly speaking you are supposed to supply the old discs with a computer since they are part of the computer package (for Macs at least).
Call Apple Customer Support 1-800-767-2775 (telephone numbers for other countries - http://support.apple.com/kb/HE57). Provide the serial number and specifications of the Mac. They may be able to provide a replacement set of discs for a fee. These discs will be for whatever OS was installed on the computer originally.
As far as your files are concerned, you can use another Mac or any retail version discs that will work in the old Mac to wipe the drive. To be more specific it would help to know what computers and which discs you actually own.
If you install an operating system (OS) on a Mac you also need to provide the discs (i'm talking about pre-OSX 10.7).
I have seen Macs sold without the original OS discs. If you do that then you at least need to provide the disc for the one on the computer. I guess you could try to sell the computer without an OS but that might be hard. If I was buying your computer I would want to see it boot, and I also might not want the hassle of getting an OS disc for it befre I could use it in any way. You'd have to come down a bit to make it sell.
Given all the above, I would still go with Snow Leopard. The retail disc itself will be as cheap as the original. The only things it might miss vs. the original will be bundled software (most of which will be outdated, though somebody might prefer older version to none) and perhaps the hardware test utility. Having a slightly newer OS might make the computer more attractive.