Postmaster processes running out of control?

I'm running PostgreSQL 7.1.3 on Red Hat Linux 7.1 and believe there is a
problem with my PostgreSQL server. I have a PHP application on a separate
server accessing the PostgreSQL server. The PostgreSQL server seems to be
getting hammered, as even simple queries on indexed columns are taking
FOREVER. When I run top, here I normally see at least 50 entries similar to
these for postmaster:

19336 postgres 9 0 92960 90M 92028 S 0.0 9.0 0:18 postmaster
19341 postgres 9 0 87996 85M 87140 S 0.0 8.5 0:09 postmaster
19355 postgres 9 0 87984 85M 87112 S 11.6 8.5 0:09 postmaster
19337 postgres 9 0 87952 85M 87092 S 0.0 8.5 0:09 postmaster

The server info from top reads:

57 processes: 54 sleeping, 3 running, 0 zombie, 0 stopped
CPU states: 54.3% user, 3.7% system, 0.0% nice, 41.8% idle
Mem: 1028908K av, 346760K used, 682148K free, 93812K shrd, 8640K
buff
Swap: 1048536K av, 0K used, 1048536K free 131796K
cached

My postgresql.conf has the following changes/additions to it that change
from the default:

max_connections = 512
sort_mem = 1024
shared_buffers = 12800

I've run a vacuum on all the tables and still see the same slow responses
from the database. Does anyone have any suggestions and/or pointers?

Thank you,

Joshua Hoover