You don't have a lot of choice when it comes to SFX power supplies, but there are some clear difference between the four PSUs we tested here. Until the second-to-last page of this review the SilverStone ST45SF-G 450W had all the signs of becoming the ultimate SFX PSU, but the relatively high noise levels it produces is a disappointment.
If your Mini-ITX system won't contain a dedicated graphics card, then all you need is a 300W power supply. Out of the FSP and Seasonic PSUs, the latter is our favourite because it's more silent. In terms of efficiency they're both about the same.
If you intend to use a separate graphics card, you should go for a 450W model. The non-Gold version of Silverstone is more affordable, more silent, and only slightly less efficient, and that's our choice for that category.
Seasonic SS-330SFD 300W
SilverStone SFX ST45SF 450W