Here's my $.02.
While it's true that the 6300 and 6400 overclock to the same frequency most of the time, how you get there can be very different. For your extra $60 for the 6400, you get a 8 multiplier on the CPU.
What this means in plain English is this: With PC6400 ram, which by spec runs at default 200MHz (400MHz/DDR2) you simply raise the CPUs front side bus from it's stock 266MHz to 400MHz and voila! 3.20GHz.

8 multiplier x 400 FSB = 3200MHz.
This will happen WITHOUT having to crazy overclock some really expensive ram to 500+MHz with crazy voltages, etc.
Everything will run in-spec. Ram running at it's defautl speed and default voltage. CPU running at a normal FSB (normal being 266/333/400FSB), etc.
The CPU will likely need to be hard-set at it's normal voltage core which is 1.320v.
The only thing you WILL need is a better heatsink for your CPU. There are many out there, but for "normal overclocking use" you can't do better (price vs. performance) than the Arctic Cooling Freezer 7 Pro. I have one and it's great.
With a 6300, you'll still need better cooling, PLUS you'll have to fiddle with super crazy settings and every system is different so you'll probably have to do a LOT of fiddling to get it stable.
My overclock took me exactly 5 minutes to do (Not including the hours of research before hand! LOL!). I set it up and it's not given me a single problem since.
For your extra $60 for the 6400, you most likely will get no more performance POTENTIAL than the 6300, but you'll get the same performance with a lot less hassle.
/My two cents, MHO, YMMV, 4.3% financing on new purchases, only on Tuesdays
