Answer: The answer to your question really depends if the boxes are in sun or shade. For a hot sunny spot, the marigolds would be a good choice, and the Starbright zinnias would be a good choice (they tend to tumble a bit and would soften the edge). The impatiens however need shade to part shade and would mix well with the polka dot plant (Hypoestes) or caladiums or coleus for example. For partial shade another good choice is fibrous rooted begonias. A nice trailer for sun is the variegated Vinca major; small forms of English ivy will also work in sun or shade.
When you build the boxes, try to make them as wide and as deep as possible so the roots will have ample room and a bit more insulation from the heat of the sun, and remember to put drainage holes in them. For more pointers on growing plants in containers such as window boxes, you might wish to look at "Container Gardening For Dummies" by Bill Marken, ISBN 0-7645-5057-8.
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