Postgres configuration for 8 CPUs, 6 GB RAM

Hi,

I am using PostgreSQL 9.1.5 for Data warehousing and OLAP puposes. Data size is around 100 GB and I have tuned my PostgreSQL accordingly still I am facing performance issues. The query performance is too low despite tables being properly indexed and are vacuumed and analyzed at regular basis. CPU usage never exceeded 15% even at peak usage times. Kindly guide me through if there are any mistakes in setting configuration parameters. Below are my system specs and please find attached my postgresql configuration parameters for current system.

OS:                                         Windows Server 2008 R2 Standard

Manufacturer:                  IBM

Mode:                                  System X3250 M3

Processor:                           Intel (R) Xeon (R) CPU  X3440 @ 2.53 GHz

Ram:                                      6 GB

OS Type:                              64 bit

Thanks in advance

Syed Asif Tanveer