Amendments will certainly add nutrients, but most important thing they do is increase water-retaining quality. The cellulose fibers in organic matter expand to retain water, then gradually shrink as they dry to leave air spaces in the soil. Great for plants, not great for keeping the walls of the house dry. I'd absolutely amend with compost when you plant your plants, but keep it (and the plants) at the outer edges of those beds. Some nice color and softening all those straight lines would look great!
A pretty flowering groundcover if you like blue flowers is Evolvulus, common name Blue Daze. The flowers are only about an inch across but the most lovely shade of blue.
Blue Daze (Evolvulus glomeratus)
It would do great on your NE facing side, as well as the south facing bed, because it looks like there's a little bit of shade on the south and west sides of the house anyway. Even if you mowed it, it will just spring back with more flowers all through the summer and fall.
