iMac kernel panic; cd/dvd drive does not boot

My early 2008 iMac does not recognize the Internal HDD as bootable. I had freezing issues (probably kernel panic issues) very common to other iMac users: screen freezes and I can only move the mouse; got to force the shut down and reboot. I tried to Repair the drive using Disk Utility from the MacOS installation DVD and after doing so, the issues will come again eventually.

Until this morning, when it wouldn't boot at all.

I want to wipe out the Internal HDD, no backup needed, and reinstall the OS from scratch. Problem is:

Tried to boot directly from the installation DVD or choosing the disk from the Startup Manager. In both cases, after a long time loading, the "spinning clock" under the apple on the grey screen freezes and a darker grey screen appears showing a "You need to restart your computer. Hold down the Power button for several seconds or press the Restart button." message. So... kernel panic.

After reseting NVRAM and all those commonly-recommended resets, I booted into Single-User mode. I proceed to Zero-erase the free space using command-line. I think something went wrong and now the HDD drive is not recognize anymore as a bootable drive.

I don't care, I just want to format it and clean-install the OS again. why is this still not possible?

After reseting NVRAM and all those commonly-recommended resets, I booted into Single-User mode. I proceed to Zero-erase the free space using command-line. I think something went wrong and now the HDD drive is not recognize anymore as a bootable drive.

Some forums/discussions point out that the HDD might not be the issue... Rather a "bad" RAM modules... 3 years ago I exchanged the default RAM and extended it with a total of 4gb (2x2 DDR2 kingstom modules)

questions:

1. Why is the DVD not being booted every single time? Shouldn't be independent from the Internal HDD state?

2. Booting this potato in Target mode and cleaning the HDD from a healthy Mac... would it allow me to install the OS from there?

3. Some forums/discussions point out that the HDD might not be the issue... Rather a "bad" RAM modules... 3 years ago I exchanged the default RAM and extended it with a total of 4gb (2x2 DDR2 kingstom modules).

thanks!