Personally I don't think you can really compare their old sound to there new sound, too different. I love both sounds, but I don't know if I could say I prefer one over the other >.>
Totally agreeing with Milly on this one man. Tarja Turunen is worlds apart from Anette and I am definitely a Tarja-fag. Having that said, Dark Passion Play definitely fucking rules.
To be honest I really don't know enough about their old sound to contrast, I came across a bunch of Nightwish in a youtube metal playlist and all the songs I REALLY liked had Annette singing. Then I got really drunk, downloaded a bunch of Nightwish from eMusic and shot off like four of five posts here on the forum blathering about how awesome they were. Which I totally don't regret doing, they are awesome.
Speaking of awesome check out Eluveitie, extreme Celtic Folk Metal for the win! I drunk downloaded them as well and haven't regretted that decision either.