Computer screen does not boot

Hello, I have a custom computer in which the motherboard supports up to 2gb ddr2 667 ram. However, a guy at the store told me that I could, however, use ddr2 800. So i bought two 1gb ddr2 800 memory cards and installed them into my pc. They worked fine but occasionally my computer got the erroneous blue screen and then restarted. I didn't mind this the first couple times until I got fed up and decided to go inside the bios of my computer and change the memory settings. I believe I lowered the maximum memory settings in the bios from 800 to 600, but my computer still got the blue screen restart after a while. I then went back in the bios and changed the memory settings to 400, but this is where the problem occurred. My computer starts up, as all the fans and everything start spinning, but then there is no display on my screen. I've tried taking out the memory cards then putting back in and what not, but this still does not work. Is there a way in which I can fix this? Help is greatly appreciated as I use my computer often. Thank you in advance.