The "unlocked" multiplier on the 6400 BE is pretty pointless. You can't make it higher, only lower. I'm able to get mine to 3.4, anymore and it just crashes. But i do have to say that you probably won't get any better performance out of another AM2 chip.
If you want to OC an AMD, I would go with a 5000+ BE or something around that range. The 6400 is already so high in clock speed, there is little head room to get much more out. But, if you wanted max performance from an AMD chip, without OCing, the 6400 would be the one to get.
non-BE can go down, not up. Right now though, the 5000BE is a pretty good deal if you already have a hsf. if you really want the 6400 (which eats more power @ probably the same speed you'll get out of the 5000), check for a BIOS update for your board. another site just had an article up about how the 125w chips can damage certain motherboards when overclocked, google and check it out.
coa can you post the vcore you used to get that 5000+ to 3.3? I've got a 5000+ currently running at 3GHz stock voltage and I've gotten it to 3.1 stock voltage as well but I want 3.3GHz because then I could use a /10 ram divisor and get my 667MHz ram.