Performance question (PostgreSQL 7.1.3)

Hi there,

I have no experiences with big databases. Now I have a database that's size
is appr. 7,6GB (HW/OS: Intel Pentium III 600MHz, 512MB RAM, GNU/Linux 2.2.19
kernel). There is a big table in it with 25318267 tuples today. If I make a
simple query (SELECT x,y,z FROM table WHERE x = something) then it takes
2m42s to get the results. Is it a good value?
 I've read the documentation and I've checked the kernel parameters (sem.h
and shmparam.h) and the parameter values in postgresql.conf and I have no
idea what could I do to get the results in shorter time period.
Thanks for any kindness in advance.

Greetings from Hungary,
Gabor