Emacs running under X will do basic formatting (italic, bold, larger text, etc.) all on its own. There are packages out there that will do more elaborate previews. Googling gives: http://savannah.gnu.org/projects/auctex
I second auctex, it's preview mode is a godsend when formatting tables and formulas. It also has dozens of helpers for switxhing environments, defining and using macros, etc.
It comes with reftex, which, by itself, makes it worth learning. Reftex automatically searches bibtex files for regexps, makes cross-references easy and a lot of other useful things.
