A quality brand 300-350 Watt PSU will run most systems (many compact and SFF systems run fairly high-end components from 200-250W PSUs - no problemo) with more than sufficient safety margin assuming your CPU is driven off the 12V rail as it should be. Look for at least 13A (more is better) on the 12V rail. Most recent PSU designs have shuffled the proportion of the total power more to the 12V side.
.bh.