Well, I pulled the chips and put one back (has to go in the top slot, btw) and ran the RAM test program and after about 15 minutes, off the network, internal temp about 160f and fans spinning at 3600-3900.
Pulled that one and put in the second one by itself with the same result. I suspect the RAM test itself is causing too much heat. I now have just the second one in and am running fine so far. I looked in the System Profiler, and the default for 10.6 on my model is 64-bit kernel and extensions: No.
I put the case back on but didn't put the screws back in yet. That way I can put both chips in tomorrow, and yes, boot with the 4 and 6 pressed and see what happens. I'm thinking that you guys have solved the problem, my question is "Do I want to have to remember to hold down the 4 and 6 or the computer will melt or just leave this one chip in?" and I think the answer is run it on one chip for now. I just want to find out if running in 64-bit kernel mode with both chips inside will cause what seems to be these heat-related or over-processing errors because if that is the case, I will send the chips back and get a set of 4's.
