I now must see that like many others, I had the idea of turning an eMac with a faulty display into a cluster node.
Interesting.
What I had in mind is putting the eMac into a rackmount case, just like what Marathon Computer did with the iMacs. Okay, I don't think I can fit it into 1U, but 2U will probably work nicely. I also found some people saying, it's possible to overclock even the 700MHz, which realy would be great to get a bit more speed. Sadly, I didn't find any specific data.
Reading through this thread, attaching the ATX power supply is what holds us back from our evil ideas. I will have to make some phone calls, perhaps I can find out what we need, although I don't think this will be to easy.
For the questions about video, the video mirror port should work (with the VGA adapter) just fine for attaching an external monitor. I would prefer the SSH/Remote Desktop way, like on the xServes