RAM / Disk ratio, any rule?

hello,

I am wondering if actually, does exists any rule to determine
the amount of RAM, according to the hardware disk size
(in prevision to use all or nearly available space),
when designing a server?

Of course fsm settings implies that when db size grows, more memory will
be in use
for tracking free space.
Same thing applies from RAM cache effectiveness: the bigger db size is,
the more RAM is needed.

Do be more specific, we have an heavy loaded server with SCSI disks
(RAID 0 on a SAS controller),
making a total of 500GB. Actually, there are 16GB RAM, representing
about 2,5% of db size.

any experiences to share?

in advance, thank you.

Mathieu