performance on new linux box

Postgresql was previously running on a single cpu linux machine with 2 gigs
of memory and a single sata drive (v8.3). Basically a desktop with linux on
it. I experienced slow performance.

So, I finally moved it to a real server. A dually zeon centos machine with
6 gigs of memory and raid 10, postgres 8.4. But, I am now experiencing even
worse performance issues.

My system is consistently highly transactional. However, there is also
regular complex queries and occasional bulk loads.

On the new system the bulk loads are extremely slower than on the previous
machine and so are the more complex queries. The smaller transactional
queries seem comparable but i had expected an improvement. Performing a db
import via psql -d databas -f dbfile illustrates this problem. It takes 5
hours to run this import. By contrast, if I perform this same exact import
on my crappy windows box with only 2 gigs of memory and default postgres
settings it takes 1 hour. Same deal with the old linux machine. How is
this possible?

Here are some of my key config settings:
max_connections = 100
shared_buffers = 768MB
effective_cache_size = 2560MB
work_mem = 16MB
maintenance_work_mem = 128MB
checkpoint_segments = 7
checkpoint_timeout = 7min
checkpoint_completion_target = 0.5

I have tried varying the shared_buffers size from 128 all the way to 1500mbs
and got basically the same result. Is there a setting change I should be
considering?

Does 8.4 have performance problems or is this unique to me?

thanks