That's a Dell trick. The only hard + fast rule on Intel duallys is that the lower stepping processor must be CPU0. This can be difficult to implement if your mobo has a floating CPU0, but most have it hard-wired.
I think that AMD does not make actual changes in the CPU during different batches. Only refines the process which affects OCability. I would not try different family AMD processors, but aside from that these machines are not that picky. That being said, I would just buy a pair of processors and be done with it, one less thing on your mind if you have problems with stability.