how to fix problem then when two queries run at the same time, it takes longer to complete then if run in sequence

I have a PostgreSQL database on a very low-resource Xen virtual machine,
48 MB RAM. When two queries run at the same time, it takes longer to
complete then if run in sequence. Is there perhaps a way to install
something like a query sequencer, which would process queries in a FIFO
manner, one at a time, even if a new query comes before the last one
running is finished, it would not give the new query to the server
before the one running now finishes? That would greatly improve
performance.

Any tips in general for running PostgreSQL on such low-resource machine?

I have:

shared_buffers = 5MB
work_mem = 1024kB

are these good values, or could perhaps changing something improve it a
bit? Any other parameters to look at?

--
Miernik
http://miernik.name/