Card for sure. I'd then focusing on upgrading RAM to at least 32GB at a respectable speed otherwise you'll get the jitters.
Yeah - optimizing is a great way to start. My advice is to get a SSD with at least 1TB and reinstalling windows. Without a fresh start - it's going to be challenging to figure out what the problems are without it. (no, you don't need to have your VR games on an SSD) The rift takes up quite a large portion on your main drive, I wouldn't recommend anything less than 500GB.