pulpfixtion

Hi, installed in my system I have 1 gig of DDR2 PC5300 667mhz. I mistakenly bought an extra gig of DDR2 PC4300 533mhz. In the old days when I was up to speed with computers your RAM had to be matched, the question I have is: with the advances in technology can I add this extra stick or do I need to change it for an identical type? My motherboard is fairly new and the processor is a 3800 dual core AMD many thanks