I'd remove the extended RAM and Airport just to rule those out, and then immediately reset the PMU and PRAM. Another test is to see if target mode and option mode happen to work immediately upon powering up, and before the fan-on-high situation kicks in.
You're not likely going to be able to effectively test external video in the fan-on-high/black screen state...generally this state is sort of a defense mechanism that the computer goes into when something is wrong, and video will not appear until the problem is alleviated, so if you don't see anything on an external screen, I don't think that necessarily means anything at this point.
The most likely cause in this situation is logic board failure. Second most likely is that removing RAM/Airport/battery and doing resets may resolve the issue. 3rd most likely is that if you go inside the machine and do exploratory surgery, you'll find something goofy (loose screws, bent shielding causing a short, etc.) that is forcing the laptop into this state. I would be tempted to disconnect the hard drive and then the optical drive, and see if that causes the machine to behave differently.
Once (but unfortunately only once) a machine in this state went back to normal when I disconnected the fan and then powered on with it disconnected. When I re-connected the fan and powered on again, the machine was fine. But again, it only happened once, and this is not typically a fix that ends up working.
Also, it's a longshot, but if you pop the back of the screen casing off, check to see that the video cable is fully seated. Sometimes loose connections can cause a laptop to panic and go into this state.