Niemi's a *much* worse goalie than Nabokov (sv% being the obvious metric, though he was admittedly outplayed in the Sharks-Hawks series) who played behind a truly stellar defense. In fact, anyone in the know (for example the writers of the sbnation hawks blog) didn't really think Niemi was more than an average goaltender. And, he's sharing the starts with Niity and given preseason play, Niity is looking much better.
Rinne will get many more starts as well, so you lose there, so Niemi << Rinne.
Franzen < H Sedin, probably but it's not going to be a blowout, assuming last year was a bit of an abberation and Franzen stays healthy. Meh
Ovechkin is way, way better than Semin. I mean, better in G,A, and then better in all applicable side categories. Not that Semin isn't amazing, but from a fantasy perspective, for a single slot, Ovechkin's almost too good.
Don't trade Ovechkin if you want to win. Or, if you hate him that much, go find the guy who has crosby and try to trade for him, but in Fantasy value (shots, PIM, etc), ovi's got much more, so do try to get something else out of the guy.
tl;dr: Don't do it, Niemi isn't that good and will split the starts, ovechkin's awesome in fantasy value.