I love those movies.
I'd recommend:
Dr, Jekyll & Mr. Hyde (1931)
Best movie version of this story
Bride of Frankenstein
The ambitious sequel to Frankenstein still gets high praise. a must-see.
The Old Dark House (be sure it's a good print)
The first film James Whale and Boris Karloff made after Frankenstein. Features the best cast ever for a horror film.
The Invisible Man
Another great James Whale movie. Made Claude Raines a star.
King Kong (1933)
First and best giant monster movie.
The Raven
Lugosi is a mad genius obsessed with both Edgar Allen Poe and a beautiful young woman. Karloff is his unwilling henchmen.
The Devil Commands
Karloff plays a scientist obsessed with communicating wit the dead. Relly far out for its time.
Frankenstein Meets the Wolfman
Starts as a great horror movie, ends as fun one. Without it, Jason would never have met Freddy Kruger, and the Alien would never have met the Predator.
Abbott & Costello Meet Frankenstein
A great horror-comedy, mostly because the monsters are so authentic.
Creature From the Black Lagoon
Great '50s monster flick.
Silent movies (Be sure it's a good print)
Nosferatu
Metropolis
Der Golem
Phantom of the Opera
As for that movie with the window curtain and the cat, the only 50s movie I can think of that was like that was The Incredible Shrinking Man, when a giant cat appears at the door of the dollhouse the title character inhabits.
Also, little people (including a woman) are menaced by a cat in Dr. Cyclops. Or maybe you're thinking of the pilot episode to the TV show Land of the Giants.