Start by testing the new 1GB stick by itself. Place the 1GB stick in
the slot furthest from the processor. Get a copy of memtest86+ from
memtest.org and prep a new floppy with it. The memtest86+ floppy is
self-booting, and you boot the computer for the first time after
the new memory is added, with that floppy. Allow the tester to run for
two full passes (there is a pass counter on the screen). It could take
a couple hours.
If the memory is error free, then add your existing stick. Place the
existing 512MB in the slot nearest the processor. With three slots,
in a single channel memory config, using slot 1 and slot 3 distributes
the loads a bit better. The slot choices make a tiny difference, and if
you didn't follow the rule, it probably wouldn't make any difference.
Now test with memtest86+ again. Sometimes, the added bus loading leads
to memory errors, and you want to find them with memtest86+, and not
by crashing Windows instead.
If the memory is crap, it is not going to matter how you arrange the
slots. So I expect your test results will be definitive.
When changing the memory, remember to unplug the computer, or at
least switch off the power strip that the computer is plugged into.
The idea is, you want +5VSB from the power supply to be disabled.
I like to unplug the computer to ensure that there is no power in
the chassis. RAM can be damaged, if sticks are plugged in while
the green LED on the motherboard is still glowing because +5VSB
is present. Taking some antistatic precautions is also a good
idea, and at a minimum, keep the chassis and the antistatic packaged
RAM at the same potential. A wrist strap makes that easier. You
clamp the alligator clip, to some shiny (conducting) metal on the
computer chassis. Or the screw on an I/O connector. That keeps your
body and the computer at the same potential, so no static jumps
from the memory socket, to the new RAM as it is inserted.
http://www.radioshack.com/product/index.jsp?productId=2103808
Naturally, people install RAM without stuff like that, but I
mention it for the sake of completeness.
Good luck,
Paul