Kernel panics; Apple hardware test error codes... Bad memory?
It sounds like some sort of RAM issue for sure, and there are a few memtest based programs out there to try and run and see if you run into problems.
Rember is a gui based memtest, and applejack is a fairly well known utility script that also contains it.

I dont know why you only got the error with a specific configuration of ram, but with 8GB of addressable space it can take a while to find the bad guy.