Dark Mantis, You were right !!
On closer inspection, the 2 Memoryboards were in fact a slower speed 533Mhz
And notice the Product number 10666 not 1066, the 10666 is PC3 - 8500F
I ran the memtest on only them and they passed fine.
But I don't like the idea of mixing different speeds, so I'll put them on ebay and use the cash to buy another 2 x 2GB sticks..
Thanks for your advice and help...
Cheers Dark Mantis