You'll need to manually set the RAM speed/timings/voltage to the recommended specs in the BIOS to run all your RAM. You should also run Memtest86+ overnight to test for RAM errors once you set the speed/timings/voltage.
You'll need to do a fresh install of Windows after replacing the motherboard. Your system will have old drivers and registry entries for your old motherboard. You can't just replace the motherboard unless you get the EXACT same model.
What are the complete system specs? Do you have a USB or PS/2 keyboard and mouse? You have to enable support for USB keyboards and mice in the BIOS on some motherboards.