No video (black screen) upon boot, is it my logic board?

Thanks to the excellent info on this site I have tried the following things that all result in the startup chime, but no video:

• Start from intall disk, holding down C
• Start from install disk, holding down D (hardware test)
• Reset PRAM w CMD+OPT+P+R
• Change RAM to known good RAM
• Using known good RAM test each RAM slot independently

I was also able to access the hard drive just fine after booting into firewire target disk mode (holding down T)

I will need to buy an adapter and borrow a friends monitor to test the external display. If I do so, what is the value of this test? Determining if it's the LCD itself vs. the logic board?

I no longer need this computer but I want to either fix it myself (I am capable of an easy or moderately difficult repair) or accurately describe the issue if I sell it as-is.

Thanks in advance!

Caitlin