Selling Comp. - What's it Worth?

Hello everyone, need some advice from you all.

I'm going to attempt to build a computer (first build) and if all goes well, I plan on selling my old computer. Two questions: 1. what's it worth and 2. what's the best way to clean up the hard drive so someone does not recover/steal my personal info/files.

Machine: Dell Inspiron 8400 purchased new in 2005. Works like a charm, not a thing wrong with it, I just want to try to build my own machine.

Has:
* 3.0 GHz Pentium 4 with Hyper Threading Technology
* Windows XP Pro Media Center Edition
* 2 Gigs of PC 5200 RAM
* Nvidia 6800 256 MB Video Card DVI/VGA and a TV Tuner
* CD/DVD RW drive
* Floppy Drive
* 8 USB ports
* Comes with a 15 inch LCD monitor, and standard keyboard/mouse
* I would keep Office 2007 installed

I see there is an auction for a similar 8400 on eBay, currently going for $152 with 1 day left. I was thinking about just watching that auction and putting my starting price about where that auction ends?

I would try to sell this on eBay, Craigs List or through the local paper - whichever will yield the highest amount.

I want to ensure my files are completely gone. Any ideas? A friend was telling me about file shredding programs you can download.

Thanks for the time!

Dell has a built in destructive restore function which came out sometime in 05. To use this do the following.
Restart your Dell.

After the Dell logo fades, you will see a blue bar at the top of the screen with "www.dell.com" in it. The blue bar is displayed for exactly two seconds.

As soon as you see that blue bar, press the Control and F11 keys at the same time, hold for a half second, then release.

This will bring up "Dell PC Restore" by Symantec. Follow the prompts to run the recovery procedure. Takes less than 15 minutes.

Be advised that running this recovery procedure deletes all user information on the hard disk drive, and recovers the c: partition of the hard disk back to the exact condition it was in when you first got the computer brand new from Dell. So before running it, be sure to offload anything you want to save.

This is perfect for your situation. Sure, the National Security Agency could still analyze the hard disk and get your secret chocolate chip cookie recipe, but average folks won't be able to recover any personal information. Be advised that whomever buys it will need to do all microsoft updates starting from the time the puter was orig. bought until now.