|The reason the RAM didn't work is because it's "ECC, Reg Ram". That type of RAM is generally meant for servers & is not supported by the majority of desktop systems.|
If I selected the correct Gateway 500 from the list, your board supports up to 4GB of PC3200 (DDR400), 1GB per slot. From the Q & A section at the link below:
"Q: Does my computer support ECC memory?
Your system does not support ECC. Because ECC and non-ECC modules should not be mixed within a system, install the same type of modules that are already in your system."
Don't waste too much money on that old system. Stick to 2GB RAM (2 x 1GB).