Performance Slowly Decreasing As Database Grows

I am using Postgresql 8.1.4 on Windows 2003; platform is Dell Precision 330,
1.8 Ghz
CPU, 1 gByte of RAM. This database is subject to 'vacuum full analyze' once
/ day.

I am watching a recently created database grow; as it grows, I am finding
that some of the performance statistics appear to be falling out of bed. The
functions I have used
to capture this information are the standard functions that come with
PostgreSQL:

date commits rollbks dsk_reads mem_reads pct_mem_hits
db_size
2006-06-19 94115102 64 553053905 13126498559 95.9600 "1674
MB"
(server restarted)
2006-06-20 4383600 26 24129603 1162150532 97.9700 "1471
MB"
2006-06-21 9179799 27 56084142 2456950412 97.7700 "1587
MB"
2006-06-22 14447111 28 89452397 3875993962 97.7400 "1710
MB"
2006-06-23 20233946 47 128309666 5553425498 97.7400 "1858
MB"
2006-06-26 34252036 4765 237496776 9024547005 97.4400 "2218
MB"
2006-06-27 40290065 403368 273062334 9156477077 97.1000 "2401
MB"
2006-06-28 46436389 870211 307983449 9260208418 96.7800 "2497
MB"
2006-06-29 52251198 1352370 348552701 9367093206 96.4100 "2575
MB"
2006-06-30 58585373 1839034 392822069 9477661079 96.0200 "2647
MB"
2006-07-03 75747589 3318388 551767504 9816311112 94.6800 "2815
MB"
2006-07-05 87768328 4524047 678763032 10076029919 93.6900 "2973
MB"
(server restarted - shared_buffers changed)
2006-07-06 4841006 326389 50641814 121507743 70.5800 "3031
MB"
2006-07-07 10711794 732981 113816094 274683161 70.7000 "3076
MB"
2006-07-10 19428743 1300797 287848710 517770353 64.2700 "3452
MB"

My questions concerning these statistics:
(1) This application monitors networks. On 6/26, it began monitoring a
number of
new network nodes (traffic increase of 40 - 50%); but the application
itself
has not been changed, either in terms of number of users, program or
database
changes, or other changes. Yet, the rollback column has increased
substantially.
QUESTION: is there a database setting that can affect this statistic?

(2) I noticed that pct_mem_hits was dropping in early July at about the same
time
that the platform started monitoring additional network nodes. On late
July
5th, increased shared_buffers from default value of 1000 to 1500, but
pct_mem_hits
continue to decline.
QUESTION: I evidently touched the wrong thing. What should I be
adjusting to
help keep the database in memory?

All other statistics appear to be normal.