What are the important things to look for in choosing a motherboard?

I have built 2 computers from scratch. I think you are on the right track. Either of the motherboards look fine to me. You are considering things correctly, CPU, motherboard to accept, consider format of the mb for selection of case. Power supply, get at least 500 watt, make sure that the number of pins for the mb connection is correct. Don't forget fans, hopefully the CPU comes with a fan. Ram, get at least 4Gb, hard drive(s) 500 Gb if you can afford. Video card must be PCI for either of these mb.