Hi David, All looks OK. I would go for a 750w Power supply - you may not need it now, but it would help to future proof - espeically if you're a game or video/photo editor. Performance video cards take more juice.
Are you sure 120Gb is large enough disk? hhhmm. A typical Windows install + office + various apps will eat up anywhere between 50-80gb YMMV.
Midi size case should be fine. Most people will get towers (typically) for the extra hard drive bays.