I'd stick with a 6400. A 7 times multi is just getting too low in my opinion. I know the p5b can do over 500 fsb but that is getting pretty high, and will you ram still be able to run 1:1? If you want a really high fsb and you have the ram for it then ye, 6300 will probably clock just as well as a 6400 (if not better as weirdly shown in an anandtech article)
Also, if it bothers you speedstep will do bugger all going from a 7-6 x multi on a 6300