Is there a reason I'd want the classic versions? I thought the remastered ones were just 1:1 the same game with better resolution, textures etc.
We won't know exactly what the differences are until we're actually able to play them ourselves, but it's possible there will be changes that not everyone agrees with. I seem to remember some people not feeling the new art for the cutscenes.
As for resolution, since Homeworld is a PC game running on a 3D engine it's entirely possible to play the original in 1920x1080, though I imagine it'll be a lot more straightforward in the remastered version.
Also, the original has had mods made for it that probably won't work with the new version. I just hope someone makes a mod for the remastered edition that enables accurate ship scaling. It's fun to see carriers getting swarmed by tiny little fighters.