Optiplex GX620 ram upgrade failure?

So I have/had a Optiplex GX620. It had 2GB of ram in it, and I wanted to upgrade to the maximum of 4. (Technically 3.5 but whatever.) I went onto Amazon and found 4x1GB ram made for GX620. I also found a compatible graphics card and a Pentium D 945. The RAM arrived first, but when me and a guy tried to put it in, the computer gave us 4 long beeps, then 2, nothing appeared on screen. We spent a long time trying to fix the issue, but eventually just gave up, the original ram that it came with doesn't even work anymore. Can anybody tell me what went wrong?