Requirements for a database server

Hi,

I have been using postgres for the last year now.  The database has grown
from a mere few MBs to over 100GB data and expected to top up 300GB by the
end of the year.  Lately performance of queries, inserts, updates has
continued to grow worse as the dataset has grown larger, even though most
queries have indexes on them, while vacuuming the database has become a
nightmare.  For this reason we were thinking of upgrading the system
hardware.

Is there anyone out there who has databases of this sort ?  Can he point me
out some basic requirements for these sort of databases (processors, ram,
storage, etc) ?  Also would it be worthwile to migrate to another database
system which is more robust with this kind of amount of data (such as
oracle) ?

Thanks
Robert