Low CPU Usage - Mailing list pgsql-performance

Hi all.
Recently I have installed a brand new server with a Pentium IV 3.2 GHz, SATA Disk, 2GB of Ram in Debian 4.0r1 with PostgreSQL 8.2.4 (previously a 8.1.9).
I have other similar server with an IDE disk, Red Hat EL 4 and PostgreSQL 8.2.3

I have almost the same postgresql.conf in both servers, but in the new one (I have more work_mem than the other one) things go really slow.  I began to monitor i/o disk and it's really ok, I have test disk with hdparm and it's 5 times faster than the IDE one.
Running the same queries in both servers in the new one it envolves almost 4 minutes instead of 18 seconds in the old one.
Both databases are the same, I have vacuum them and I don't know how to manage this issue.
The only weird thing is than in the older server running the query it uses 30% of CPU instead of 3 o 5 % of the new one!!!
What's is happening with this server? I upgrade from 8.1.9 to 8.2.4 trying to solve this issue but I can't find a solution.

Any ideas?
Regards
Agustin