Ok, if you got that HSF then just use it. You'll be fine there.
As for getting the 6400, it won't help you with games if you overclock. Maybe a couple frames, that's it. But it will help with other things.
So, getting the 6400 depends on:
1) your budget
2) how high you want to overclock
3) what you use your computer for that will benefit from the overclocking (ie--video ripping, etc)
Since you're a beginner to overclocking, the 6400 will definitely help you with its higher multipler, and let you get a better overclock easier. So if you can afford the $40 extra, I'd recommend it. If you can't afford the extra $40, then the 6300 will still do fine.