RAM cartridges

I am a new MSX owner (the yamaha CX5M) and a total amateur. I was curious as to whether there are RAM catridges that I can plug into the rear slot of my computer that will enable me to do things like install dos, etc. Any help would be greatly appreciated. So far all Ive been able to acquire is a 4k cart, and I'm not sure what I can use it for if anything. Anyone else using the CX5M, please contact me. There don't seem to be many of us out there...