memory tuning

I have 2 GB of RAM on this machine and a table with 2.9Million rows.

The most I'm getting reported on memory usage for postgres (7.4 can't upgrade
just yet) is 2.6%.

I have set the following parameters:

shared_buffers = 3000           # min 16, at least max_connections*2, 8KB each
sort_mem = 262144               # min 64, size in KB
vacuum_mem = 262144             # min 1024, size in KB


But I'm not utilizing much memory and things are running frighteningly slow.
I did run a VACUUM ANALYZE today with no incidents so I wouldn't expect that to
be a problem.  I don't think there have been more than 10^5 transactions since
then (assuming that's not too many?)

tips?