While the 1.8GHz E6320 has been a pleasant house guest, just a tenner more nabs this E6420 with nearly 300MHz extra on the clock. The higher multiplier of the E6400 gives it a slight edge on mobos incapable of monster FSBs, drawing it very close to the pricier E6600 and beyond.
Still, justifying the £35 extra over the lower specced but able-to-catch-up-through-clocking E4300 is tricky.
The sudden appearance of a '2' in the middle of the E63 and E64's names is because these are revisions that bump the cache from 2 to 4Mb, the latter previously reserved for the pricier E6600 and up. The performance difference is not vast, but it's a welcome boost.