Both PlecoDict and Oxford E&C work quite well on the Treo 600 (I've been using one since we started development on PlecoDict) - actually there is a bug that can occasionally cause crashes when entering text, but it mostly affects the 650 and it's already been fixed for the next update.
I just switched to a 650 in the hope of getting PlecoDict running similarly well on that, but it may be rather unstable in the first beta release at least - the basic search features etc should work pretty well, but you may encounter problems with flashcards and particularly with custom dictionary entries. We're certainly planning to do what we can to fix those, though.
In general, I would not personally recommend the 650 to ANYONE at the moment (in fact I should probably update the "which Palm do I buy" page to reflect my recent experiences) - my 650 at least seems to be horribly buggy, and even without any Pleco or other third-party software installed it frequently crashes, resets itself, freezes up, etc. And when I do install software on it it just gets even worse. Hopefully palmOne will come up with a firmware update soon that will fix a lot of these problems, but for right now I really don't think the extra features are worth the aggravation.