PC6400 and PC2 6400 are the same thing. PC6400 is the correct RAM. I would normally not advise mixing brands of memory, but there is no harm in trying it.
If you are building a new PC why are you using DDR2? DDR3 is the current standard.
No, you're safe to use the two together, if they're compatible. [This is never a sure thing, it's not something you can look up]. If they aren't, all that will happen is your PC will be unstable, and randomly crash, which will be annoying.
