I am going to sell my old Mac desktop, how do I delete everything personal, permanently?

What version of Mac OS X, do you have the installation disc still, and hard drive or solid-state drive?

The exact options differ from version to version, but in any case to be 100% thorough you need to erase the entire disk and then reinstall Mac OS X. Simply deleting files and/or partitions doesn't delete the information. The information remains until it is overwritten.

Solid-state drives are notorious for erasing and you'll have to use the utility provided by the manufacturer to guarantee it is erased.

Mac OS X Version 10.5.8

I have the instillation disks still.

Not sure about your last question...

http://www.mactip.net/how-to-securely-wipe-your-mac-hard-drive-before-selling-it/

http://www.wikihow.com/Wipe-a-Mac-Clean

Thank you

Next question, how much could I get for it?

Mac OSX 10.5
Processor: 2x2.8 GHZ Quad Core Intel Xeon
Memory: 2 gb 800Mz ddr2 FB-DIMM

can it be upgraded to mountain lion, if not mavericks?

I'm not saying you need to upgrade it to that, but is it an option for a buyer? The problem is that 10.5 is already EOL and has no software support. 10.6 is going to EOL soon and will run out of software support quickly after that (Apple almost instantly drops support for things like iTunes and newer i-devices after EOLing an OS). So if it can't be upgraded to a newer, supported version of Mac OS X, it's value is going to be significantly less.