Hi folks. I recently upgraded to a Gigabyte 770 motherboard and an ATI HD4870. I am looking to get a Phenom II 550 now, and I would like to get some new RAM to replcae my 2GB of PC6400 DDR2. My motherboard is DDR2 (couldnt afford the jump to a DDR3 system) and I am looking to get at least 4GB of RAM. Should I go for PC8500 or 6400? And if i kept my old RAM in as well, would the newer RAM be gimped?