AMD K6 2/500 QUESTION

I upgraded my amd 300 to the amd k6-2 500. I set all the jumpers correctly and my computer reads only a 400. I checked and rechecked the jumpers and they are correct. I have noticed that the processor runs on an ATX power supply, but my computer tells me im running an AT. Will this effect the performance of the processor? If so, how can i change my computer to an ATX power supply? ZDNET.com doesnt offer processor benchmarks anymore, so cant see what it is really doing.

Thanks
Paul