IMO get the e6600 as there is quite a bit of performance difference with the added cache, however if not, I'd just go for the 6300: it's really cheap, and OC's like mad.
id get the E6400, the cache dosent really do that much, a E6400 oced to around 2.8 gets about the same permormance as a stock X6800 or E6700, i dont reallly remember.
Hm, I have an E6600 and it hasn't done too well on a DS3, but Idiotec has an E6400 and so far it's done better than mine in overclocking. We both have DS3's.