There is no "too old"; probably the processors must be not right (even if they have the same number and specs; I have noticed that some cpu were sold as compatible with mac, but they were not.
I took mine from a user that had them on a mac, so that's why mine works without problems.
I am not a microprocessor engineer so I have no idea why would you get that kind of problem, but probably someone on the forum can give you some pointers to see what is going on (like a low level log that you can check to see what is going on before that you even boot the OS.
Check the info on the surface of the CPU and compare it with what you have; you may get some hint from what is on the CPU.
BTW what OS do you have? Do you have original Apple memories or aftermarket? Since the error appears to be on the memory I would check with different memory.
And you cannot update the firmware, the last version of the firmware was 1.1; so if you try to put 2.1 on it you may mess up your computer....I heard of some people that were trying to install new EFI and boot to have the fll 64 bit support (and install w7 64 on bootcamp), but I never heard of someone that was successful.