Sounds just like my front yard. Actually you have the ideal conditions for many plants. Morning and afternoon sun are great and if you run a garden soaker hose under the mulch, you shouldn't have any worries about water.
Everything grows in my garden with similar conditions.
Astilbes make a wonderful addition and only really need to be watered diligently for the first year. Hostas will grow any where and there are a number of new ones out there now so you don't have use your Mother's (although if you are on a budget that is not a bad idea).Heuchera, you know them as coral bells, also do well but require more water if you are getting hot afternoon sun in the summer.
Stay away from ferns and other woodland plants as they like moist conditions and remember that your yews are in love with acid soil so if they are thriving you should stick with plants that agree with this.
You may also want to extend your border in front of the yews with a softer edge. Curved edges not only make your border more appealing to the eye, but are easier to mow around. With the extension of your border you can see if the sun comes closer to the outer edge and plant some other goodies here.
Just remember to plant in clumps of odd numbers, 3 is good, and not in a straight line. You are trying to correct that look.
And don't forget annuals for a colorful summer. There are so many that will grow in those conditions and you are in a great zone for many other plants that I can only dream of in New England.
Go to the websites of nurseries that sell online, but don't buy from them, just get ideas. Make lists and drawings and head to your local nursery where you can get healthier plants already established for your area and the sound advice of a reliable nursery person.
Good luck.

Thank you wildflower 12 for your suggestions! That's a good point about the acid soil. I love ferns but I think you're right about the moistness factor...I'll try the ferns on a different side of the house that more shaded and more moist. I like Astilbes a lot and think I'll try those along with some hosta, and some annuals for added flowering. Thanks also for the three-dimensional planning tips...I'm looking forward to making that straight line more organic-looking and curvy.

-Topie