The SIMMs that you use is could be to high of a density. If you have your motherboard manual you will see what density of SIMMs that you can use.
You can also run memtest86 to find out if the memory is ok.
If memtest86 saids its ok and it found 48 MB, try specify mem=48M to the boot loader. If it works, add it to the append in LILO or the kernel line in GRUB.
memtest86 -
http://www.memtest86.com/
