PostgreSQL performance

Dear Master

I have already installed postgreSQL-7.3.1 in debian woody kernel 2.4.18. I got
some curiousity.
1. I don't have wal_files in my postgresql.conf.
2. There is no SHMMIN,SHMSEG,SEMMNI,SEMMNS,SEMMSL,SEMMAP,SEMVMX in
/proc/sys/kernel because I want to tune up my postgreSQL server.
3. I used HP LC Netserver P133 RAM 128, and I still used postgresql.conf
(original). When update 2 tables in the same schema and I do it 30 times with
the same query. The tables have 45 fields and 19000 records. Look at the
bellow. I used vmstat command :

(at first when I turn on my PC)
procs memory swap io system cpu
r b w swpd free buff cache si so bi bo in cs us sy id
0 0 0 0 85724 1360 27652 0 0 13 1 104 7 1 0 99

(at firs time I used my database : psql csa)
procs memory swap io system cpu
r b w swpd free buff cache si so bi bo in cs us sy id
0 0 0 0 52848 1464 59396 0 0 10 4 104 5 1 0 99

(at first time I update my table)
procs memory swap io system cpu
r b w swpd free buff cache si so bi bo in cs us sy id
1 0 0 0 40772 1476 70052 0 0 10 9 104 5 2 0 98

(at 11 times)
procs memory swap io system cpu
r b w swpd free buff cache si so bi bo in cs us sy id
0 0 0 0 6536 456 106920 0 0 35 289 112 19 31 4 64

(at 12 times)
procs memory swap io system cpu
r b w swpd free buff cache si so bi bo in cs us sy id
0 0 0 132 6548 456 106924 0 0 34 303 113 19 32 5 63

(at 30 times)
procs memory swap io system cpu
r b w swpd free buff cache si so bi bo in cs us sy id
0 0 0 972 6436 488 108080 0 0 99 385 119 26 25 5 71

In my postgresql.conf : autocommit=true (by default) .....so if there is
autcommit mode why potgreSQL still used memory and virtual memory to get
transaction.

TIA

Best Reagards
Aris Wendy