I wont watch any movie before the 80s. People call them classic (and Im sure they are) but technology has moved so much since then that watching anything prior (like the original Star Trek) just looks so fake and very aged.
I do understand where you're coming from. I can't stand some of the 40s & 50s sci-fi movies my dad tries to show me. But there are some great movies before the 80s, still:
-Dawn of the Dead
-The Day the Earth Stood Still
-Escape From the Planet of the Apes
-Halloween 1 & 2
-Jason & the Argonauts ("Clash of the Titans" is it's spiritual successor)
-The Last Man on Earth ("I Am Legend" is a remake of this)
-Night of the Living Dead
-The Omega Man (An update to "The Last Man on Earth")
-Planet of the Apes
-Star Trek: The Motion Picture
-Star Wars: A New Hope
-The Texas Chainsaw Massacre