How do I obtain 'sharp' text? It's always poor quality when I use high (600) dpi to png.

Also thick sigmoid (S-shaped) paths are a bit jagged in the same document that a straight path/line is clean; any solutions?

Hi.

"Fuzzy" is relative, guess you refer to anti-aliasing.
You can switch it off in the document's properties (Shift+Ctrl+D) since 0.91.

I'm not sure about this. But if you use higher DPI when exporting to PNG, all it does is change the size of the overall image. So the result is bound to be a little pixelated. At least until you use it somewhere else, either in a frame of its original size, like on a webpage, or you print it.