Can't bootup..stuck grey screen

I could go to openfirmware mode, and also I could use mac os 10.5 CDs to proceed to a certain point where "cannot be installed on this computer" comes up... at this point, I could use disk utility to at least see my HD model/serial number, but it says not mounted.

So my question is,

based on the above symptom, what's the likelihood of this being a logic board failure?

(I'm asking this question because I've tried swapping to a new hard drive and the same "cannot be installed.." message came up).

If diskutility at least identifies my HD, is my logic board OK?

Thanks in advance for any advice.

-genius bar sucks