Disappointing performance in db migrated from MS SQL Server

Hello all. I am in the midst of porting a large web application from a
MS SQL Server backend to PostgreSQL. The migration work is basically
complete, and we're at the testing and optimization phase of the
project. The results so far have been disappointing, with Postgres
performing queries in about the same time as SQL Server even though
Postgres is running on a dedicated box with about 4 times the clock
speed of the SQL Server box. For a chart of my results, please see
http://leonout.com/pggraph.pdf for a graph of some test results.

Here are the specs of the systems:

SQL Server
Dell PowerEdge 2400
Windows 2000 Advanced Server
Dual Pentium III 667
2 GB Registered PC133 SDRAM
MS SQL Server 2000 SP2 - shared database (although to be fair, this app
is by far the heaviest)
RAID 1 for system / RAID 5 for data (10k RPM Ultra160 SCSI drives)

PostgreSQL
Dell PowerEdge 2650
RedHat Enterprise Linux 3.1
Dual Xeon 3.06 GHz (Hyperthreading currently disabled)
4 GB DDR SDRAM
PostgreSQL 7.4 - dedicated to this app, with no other apps running on
system
RAID 5 (15k RPM Ultra160 SCSI drives)

The database is about 4.3 GB in size.

My postgresql.conf is as follows:

max_connections = 50
shared_buffers = 10000 # min 16, at least max_connections*2,
8KB each - default is 1000
sort_mem = 2000 # min 64, size in KB - default is 1024
(commented out)
effective_cache_size = 250000 # typically 8KB each - default is 1000
(commented out)
geqo = true

lc_messages = 'en_US.UTF-8' # locale for system error
message strings
lc_monetary = 'en_US.UTF-8' # locale for monetary formatting
lc_numeric = 'en_US.UTF-8' # locale for number formatting
lc_time = 'en_US.UTF-8' # locale for time formatting

I hope that someone can help with this. Thanks in advance for your help!

Leon