Get on the internet and do a search for memtest86, and make a memtest floppy.

Untar the memtest86-3.0.tar.gz file by using (assuming you also get version 3.0 if you get a newer version use that version number:

tar -zxvf memtest86-3.0.tar.gz

then:
cd memtest86-3.0
and (as root) place a floppy disk into your floppy drive and issue the following command:
dd if=precomp.bin of=/dev/fd0
which will write the precompiled binary to a floppy (Note: /dev/fd0 is f d zero)

then put your second stick of ram into your box and boot to the memtest floppy, so it can test your ram.

HTH

~rock