Postgresql.conf - which param for in memory keys and tables?

Hello,

Which param will I need to increase when I want postgresql to store the most
requested keys, maximum 500 MB, in memory and to store the mos requested
datasets, maximum 750 MB, in memory.

So when there is a select statement the memory will be consumed instead of
the slow disks.

I would guess these are parameters like work_mem or maintenance_work_mem,
but I am not sure.

Thank you,

Christian