509MB is OK since you have on-board VGA.
Try one stick in the first socket, some on-board VGA require first socket to be populated.
Go over everything slowly, all the cables.
You might have disconnected the hard drive data and/ or power cable so it is looking to boot from CD with nothing in it then from Network.
You might be at a point that requires a BIOS reset.
There is a difference between a faulty and dead RAM, all I have been trying to do is try or test one at a time!
Something is wrong for sure, but I bet you anything, it would have taken less than 30 minutes on a work bench to figure it out.
Again, buy RAM in pairs for best performance
We can either do this right and have a proper hardware and software setup or just change a component or drivers … Band-Aid Treatment
I do this everyday, 7 days a week some weeks, I'm not just coming up with ideas to have fun with you at 3:00 AM, and I hope you realize that.