Database Server is too Slow

Hi!
I need help, i mounted a Postgresql server on IBM netserver with RAID 5 134 GB, 2.5 GB de RAM and two Xeon 1.4 ghz.
I installed postgresql 7.3 in this box, my database have over 25 millons of records, and everything is slow.
i optimized with indexes,vacuum analyze and tuning up the kernel for shared memory usage.
but the server is too slow when:
- create index for any table with more than 5 millions
- vacuum takes over four hours in this type of tables.
- select max (field) from table where field2=xxxx takes a lot of time.

explain select max(numoble) from obligacion where periodo=200100;

QUERY PLAN

-------------------------------------------------------------------------------------------------

Aggregate (cost=6160676.57..6160676.57 rows=1 width=16)

-> Index Scan using obligacion8 on obligacion (cost=0.00..6156384.72 rows=1716743 width=16)

Index Cond: (periodo = 200100::numeric)

(3 rows)

please, i need help about this, i read a lot of documantation for tune up a postgresql server and still having problems.
anybody have a similar problem? how to solve it???

thanks in advance.