By the fastest memory for your machine you can afford. You can use separate disc in any version of AE for cash. The faster the drive, the faster the playback. The sowdown comes in processing the effects. More you have the longer it takes to render. Writing to the drive and reading from the drive is usually the smallest component of the time required to set up a preview or do a render. If your project involves a lot of footage then you are going to see improvements if the footage is stored on a very fast drive, or better yet, an array.
I understand what you say about ssd and chace but I don't understand about ram.
There are a lot of ram...... and more is the speed of ram and more is the latency.......if you buy 2133 mhz with latency cl9 you will pay a lot of money........ my question is about 1600 cl9 and 2133 cl11 which is better for a fast ram preview.
I don't know. I've never tested ram. I just always buy the recommended. I cannot imagine that it would make very much difference. You're never going to get a 50% improvement in preview speed by just changing the RAM specifications.