Yea that pc 6400 stuff most likely wont overclock very well or at all. Your most likely gonna have to runn both pairs at the slower speed of the two, which is ddr2 800.
Start off by running them at ddr2 800 and see how it goes, perhaps giving them a run or two on memtest. If you pass, you can slowly increase the frequency until you get an error, at that point you can either back it off to the last known stable setting, or you can try upping the voltage a little.