Beep codes - new RAM

I have a 20" Intel iMac 2.0Ghz with 1GB of RAM. I just bought a 2GB stick of GB Micro matching RAM, popped it in, and it worked great - for about 12 hours. I was using Parallels when I got a the "your computer crashed, you need to shut it off" screen. After this, I can't boot with the new RAM installed anymore. I get 2 beeps followed by a repeating set of 3 beeps. I've read that this means "incompatible RAM" followed by "no good banks." I tried re-seating the RAM, switching slots, etc. I've got the original RAM in the opposite slot now so I know both slots work.

Is this RAM bad or is it truly incompatible? The specs appear to match exactly (GB Micro "AE" 2GB DDR2-667 CL5, PC2-5300S-555) and it worked great for a while.