spreading the DB?

hi,

what do u do if the DB become big and u have to spread
it into two or more computers.
Links and Docs are ok too ?

For example what if u do if u want to split the DB on two servers (at desing stage), but
u have common data such as user info,address etc... if u decide to do it that way
u need to syncronize common data in some way, what do u do.

tia