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