My powermac G5 will not accept additiona memory


I have recently purchased a new G5 Power mac Single 1.8 GHz processor with 4 x 128 meg memory sticks.

I Have tried to upgrade the memory.
I took out two of the 128 memory sticks and put two new 512 sticks in.
Upon reboot I get two beeps (which does indicate incompatible memory).
So I took out all the memory and put the 512 sticks in on their own, still lthe same error message.
I have tried another two completly different memory sticks (same spec) and get the same error,I know that this memory is the correct memory as I checked on the kingston configurator. (KTA-G5400/512).

This machine uses dual pc 3200 memory sticks.

Any Ideas ? my 4 128 sticks work fine so it is puzzlng ? is this a possible hardware fault?
Any thoughts would be appreiciated.