securing the sql server ?

Hello ppl,
any one can tell me how I can secure linux server with database
postgres for example ?
Im thinking to make a cryptfs file system and to deploy database over
the cryptfs. The problem
here may will be when front end need any data for in/out cpus of the
server will aways
decrypt/encrypt data and performance will be very low.

I remember a few months ago some one ask similar question about how he
can crypt data that is
stored on database and problem was the key. Key is stored on the same
server if some one
get access can decrypt data.

Any one have some ideas how to make something like crypt bubble and to
store database there ?
Or something else ?

R.