I believe the Morrowind Graphics Extender, which has by far the largest impact on graphics, requires the two expansions, so you'd be limited to just texture replacers without things like http://img217.imageshack.us/img217/3811/birdvvardenfell.jpg.
I'd definitely recommend finding the expansions, finding a GOTY copy of Morrowind is probably the easiest. They're very good and many cool mods rely on them.