As long as the keyboard isn't "adapted" (like a PS2-to-USB adaptor) or isn't a wireless keyboard it should work.
Did you follow the steps in the tech note I linked to? It says to reseat the RAM, and if that doesn't work, then remove sticks and add one known good stick and try different combinations.
It then goes on to suggest reseating the processor, as it could be a processor problem.
Did you try those things? A hardware beep at startup is usually pretty accurate at diagnosing the problem, and that tech note outlines the parts of the system that could be causing the problem. My suggestion is to follow the tech note, with the addition of resetting the CUDA/PMU motherboard switch first.