If ur thinking of getting some wc'ing - I'd suggest getting involved in that with the main part of ur spend.
(I'm an nVidia fan, so I'm bound to say 275 if there's a choice. 285 or whatever)
Wc'ing tho, if u get into it now, use it on ur 6400, which means u'll get 775 plate etc, and the beauty is u will reuse the majority of the kit (cept the plate if u change sockets obviously) whenever u upgrade in the future.
Good thing to get into wc'ing, it's very enthusiast - and there's lots of help on OC3D for it. Once u've gone wc, u'll probably wc everything
An oc'd E6400 will play ur game no problem. OC'd under water it'll be great !
(plus it's a cheap 2nd hand chip if u manage to fry it