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.
Slow computer got you down? Maybe a hardware upgrade should be in your cards, if so this guide to the Fundamentals of Upgrading a PC is an invaluable read. PCSTATS Tips