Butterflies like nectar plants that have lots of small flowers clustered together. The old reliables are zinnia, native milkweeds, purple coneflower, Mexican sunflower, and my personal favorite, Lantana. Some of these are too tall to plant under Buddleia, and Lantana may not be hardy in your zone, so you might consider some of the Salvias, Dianthus, or even a bed of parsley.

If you plant milkweed, don't be surprised to find caterpillars eating the leaves. The milkweeds are not only attractive nectar flowers for most butterflies, they are host plants for Monarchs. You may want to plant other host plants (plants on which caterpillars feed) for other species of butterflies. If you want to induce butterflies to lay eggs in your garden, check to see which butterflies are common in your area, find out what they eat, and plant it. Here's a good place to start:
Butterflies and Moths of North America.

Good luck. Let us know how it goes.
geni

My Mom-in-Law has a white buddleia, and the butterflies COVER that thing! Your butterflies might too, might not. They can be fickle creatures. Butterflies are not touching my buddleias this year.

In the FAQ section there is a list of good nectar plants.

What state are you in? Are there local native plants that attract butterflies? Sometimes those can be used in cultivated gardens.

What colors/heights would you like?

Have fun with your new garden! It'll probably the first of many!

Oh thank you for replying. :)
I would like to keep this garden neat, so something that's not going to be invasive would be best. I guess something low-growing would be nice, because I'm going to attempt to train the buddleia into tree form. I was considering edging the back of the bed with monarda of different colors, but I haven't had any in years and forget it's growth habit. Will it stay put, or take over the entire bed?
I was also considering starting a wild-ish butterfly garden as well, across the yard from the currently developing one. With milkweed and other less...oh how to be polite, here..."cultured" plants.
I do have a lot to learn! I'll start research after work today. Thanks again! :) ~JoAnn

For low growing plants: how about Stokes aster, asclepias tuberosa (nectar and larval food for Monarchs), coreopsis, verbena bonariensis (can get a little airy with its stems), blanket flower (gaillardia).

I have a few different kinds of Monarda in my garden, and it tends to spread in some, and in another one, keeps a pretty tidy clump--slowly spreading, but I welcome it. The butterflies do love it so much!

If you want Swallowtails, definitely plant some fennel and flat leaf parsley. (the flat leaf grows bigger/bushier than curly parsley for me). This can be in your 'neat' garden or in the 'wilder' garden. IMO, Swallowtails are the prettiest butterflies. So far I've only had Black Swallowtails, but I'm pretty certain I'll end up with some Tiger Swallowtails this year too.

On Sunday I was showing my daughter's friend a stand of Echinaceas with some Painted Ladies on the cones. There were 6 of them, but as we watched another 4 arrived to feast. This is a must for a butterfly garden as far as I'm concerned. Oh, and Buddleias too, but you already have those.

Just a note, out of all the Echinaceas I have, the butterflies feed mostly on the purples/pinks.