small form factor build, cooling concerns

I'm building a small portable desktop based off of a microATX mobo. The cpu cooler and vid card will be ducted, how much airflow do I need to have over the rest of the mobo? I would prefer the system to be enclosed- then dust wouldn't be as much of a problem. Is this possible? Would could the system be passive (besides cpu/gpu) if I made vents properly?

Thanks.