PG slowdown

Hello,

My hardware

CPU: Intel 3 GHZ X 4
Memory: 9 GB
LINUX

My conf file are as follows:

shared_buffers=10000
wal_buffers=128
vacuum_mem = 256000
sort_mem=32768
max_fsm_pages = 3000000
max_fsm_relations = 32768
checkpoint_segments = 2048
checkpoint_timeout = 1800
checkpoint_warning = 300
effective_cache_size = 262144
random_page_cost = 2
default_statistics_target = 250

Some of my queries that used to run within a minute, before I changed the
"default_statistics_target" from 10 to 250, now runs for 20-30 minutes. Some
queries did see improvement in their run time.

I have two questions:

1. Why did the other queries slowdown? I tried to tune them, as their is a
join between two tables and it reads all the rows, I forced it to do a seq
scan rather than index scan but it quadrupled it run time. Why?
2. How can I disable the logging of updates? What are the pitfalls? When an
update or insert happening there are lot of logging going on and the system
writes messages like this "2006-07-11 10:08:54 [2330] LOG: checkpoints are
occurring too frequently (182 seconds apart)
HINT: Consider increasing the configuration parameter
"checkpoint_segments"."

Thank you for the help.

Abu