RAM Issues

I have a computer with two sticks of 512mb DDR PC2100 ECC memory. I recently bought one stick of 1gb DDR PC2100 ECC memory for it, however when I put it in, the computer fails the POST and continuously beeps. I thought I had a bad stick, so I got it replaced, and it's still beeping. I've tried using different slots, and it still doesn't work. The computer works fine without said stick.

Any ideas?