New ram at lower speed

I just got new ram (2x2gb, 800mhz) my old ram (2x1gb) is at a higher speed  1000mhz, I've been getting the BSOD, I know it's the ram, but I was also wondering did I have to change something in the BIOS?

Obviously you have to operate all the modules at the frequency supported by the slowest modules, ie 800MHz. Same for the latency settings. Use the ones of the slowest module.

Also some mainboards seem to have stability issues when all memory slots are equipped. So if the crashes continue even with the correct settings you might want to try a BIOS update. If that doesn't help you might be out of luck.