So I have a 1TB WD Blue and it's too slow for my 4770k @ 4.4GHZ along with CUDA acceleration because when I'm rendering or compressing a video and look at task manager it's says disk is at 100% usage and CPU is only 30/65% usage and ram is only 25-55% usage. It's even like that without CUDA. I want to make the most out of my PC so I was wondering if I can just buy a 64gb ssd and use that to cache my current hard drive or will just buying the seagate ST1000DX001 SSHD be faster performance wise and easier to set up? So please give and opinion SSHD or SSD cache. If you choose SSD cache please tell me how to use it as a cache. The reason I'm not going pure ssd is because my budget is $110 and I don't want just want 128GB of quick working space.
Caching is only useful for speeding up the loading of the same file, over and over, like your OS, so neither of your listed solutions will really speed rendering up.
For fast rendering, it is important to have AT LEAST 2 drives: one to read from and another to write to. If you try to do both tasks simultaneously on only one drive, it leads to the problems that you are seeing. Some even recommend 3 drives.
If you are otherwise satisfied with the WD Blue, then get another to separate your read/write activities. I can highly recommend the Seagate ST1000DM003 1 TB HDD that I have in my system and I believe that, according to manufacturer's specs at least, it is slightly faster than the WD Blue.