I wouldn't say that Gish is "grungy" at all. I'm listening to it right now and there is a lot more in the way of pretty/post-shoegazer stuff than "grunge".
Actually I wouldn't say any of their stuff is "grungy".
RE: Mellon Collie, I think approximately half the album is great. I agree that "XYU" is fantastic but I really hate the ultra-muddy production of "Bodies". As far as the singles, "1979" is an odd bird. It should be boring as shit, but it manages to engage me. "Bullet With Butterfly Wings" is just a million kinds of rock, I don't care what anyone says. Same with "Zero" - even the high-school-kid-discovering-Nietzsche lyrics can't detract from its sheer awesomeness. I honestly don't even remember what "Tonight Tonight" sounds like.
I do know I listened to the second disc a lot less than the first one.
If Billy is such a shitty singer, why does he sound so awesome on Gish? I'm not believing that "piecing the vocals together syllable by syllable" thing either. I've heard the "Billy played all the bass and guitar parts" rumor before, but not the vocal thing. Sauce?
Oh, and I forgot to add that it's worth tracking down the CD-single for "Zero" because of the really awesome 23-minute song that's just a bunch of fragments of unfinished/unreleased tracks.