Is this PSU powerful enough?

Its an FSP 90w pico PSU and will be powering the following setup (WHS rebuild):

Gigabyte GA-D525TUD board (Atom)
1x DDR3 stick
3x standard 7200rpm SATA HDDs

Thanks in advance!

Probably. I've used that FSP PSU before and had an issue with it, but I was trying to power up a full-fat desktop rig (albeit heavily undervolted). IIRC It only supplies 5A on the 12v line, so it's really only a 60W PSU, but that should still be plenty to power an Atom board, as long as you're not planning on adding a high-power-draw expansion card :)

I think you can stay easy, Atom is not watt-hungry :) that's the way it's used in netbooks, NAS and HTPC