Yes... if you would purchase another 1GB (2x512) or 512MB (2x256) of Crucial, I believe you would be ok. I had the exact same problem. When running the two different sets of memory, I had problems. When each set was ran individually, it didn't work. My guess is the PNY isn't quite correct. Both sets of DIMMS that I had listed the same cache latency on the label, however the SPD chip reported differently between the two. (I was able to tell this in the Apple hardware test) When Apple sent the new set of DIMMS, all four DIMMS reported exactly the same, and my problems went away.
At this point, my opinion is you have two sets of incompatible RAM. If you purchased the RAM from Crucial directly, I would trust their recommendation over the PNY RAM you purchased. I DO NOT think this is a logic board problem.