Are you most worried about load times, or frame rates, or quality settings? Where is your biggest priority, assuming of course that some combination of all of them would be nice too?
Without question your 1TB HDD is probably the first thing that needs to go and be replaced by an SSD, which is much faster, but the problem with that is that SSDs are not cheap if you want one that is large enough to hold any large games, and if you use the SSD just for the OS and applications, and put your games on the HDD, then you're still right where you started with slow load times. Having a smaller SSD (250-500GB) for the operating system and another 1TB SSD for games to live on is pretty much a standard configuration these days but then you're talking about a 200 dollar investment without even a nod in the direction of the graphics card or CPU.
Honestly, unless you are having seriously long, laggy load times to the point that they are really bothering you, maybe you ignore that for now and go elsewhere on the upgrade. But if that IS the biggest thorn in your side, then ditching the HDD or relegating it to backup duty of some kind for OS or file backups, might be a really good idea. There are few things that can immediately pick up the snappiness of a system like going from a HDD to an SSD, but it will only help your games if the games are on the SSD too, or on an SSD of their own.
Also, if you get an SSD for the operating system and applications, then it becomes a question of how much space, not counting your game folders, is being used on your current 1TB HDD because the SSD needs to be about 25% larger than whatever the total amount of space you want to move to the SSD is going to be, if you want to clone the existing operating system from the HDD to the SSD. Otherwise, you're looking at starting from scratch with a clean install and if it's been a while since you've done one, then it might not be the worst idea in the world ANYHOW to just DO a clean install if you decide to put the OS on a new SSD.