By "turning point" I mean the actual song structures were becoming much more varied and experimental. It seems to me the stuff up to and including the Gallery was somewhat formulatic. I'm not saying that in a bad way, they invented this formula, they were very good at it, and a lot of people (myself included) really enjoyed this formula.
As for getting all their cds, I have them all already (well, except Enter Suicidal Angels).
I really like the Swedish melodic death sound. I somewhat prefer early In Flames to early DT, but I much prefer recent DT to recent In Flames.