Prefix question

Hi!

If I could get a Prefix/HiCap for my LP12/Geddon/Tok4/Troika (I still don't have MC-Boards...) - what type should I take? What is the difference between K and S (and ...X?), and: can I use another cartidge in case I want upgrade the Troika, or is it only for Troika/Arkiv? Or do I never need to upgrade Troika? :-)

Thanks for any advice!
Jun