My friend and I are attempting to take a school computer (Dell Optiplex 980), and using our HDD, GPU, and PSU, take it to a competition for video editing. All of our components work, but we're still getting a flashing 3 4 lights when turning it on. We've taken RAM from another Optiplex 980 that we know works, but still it doesn't work. We're thinking it's the motherboard, and have found a cheap one to replace it, but we were wondering if there is anything else we can try before buying it. 

Thanks in advance for any help!