I wouldnt trade Pierce for Smith... I would trade Allen or KG or both but Im not sure what Atlanta would be interested in from us. Aside from Rondo or Pierce Id throw a package with anyone for Smith.
to have the cap room to sign someone like Howard you have to trade Pierce for Smith as if you don't a lot of the cap room is used up by Smith.
Frankly I don't think Atlanta takes on Pierce's contract straight up for Smith. They might do this however as it shaves money elsewhere.
Pierce, filler (anyone outside of Moore as he isn't enough salary) for Smith, Williams
Marvin is essentially Jeff Green (pros and cons a bit different but essentially the same player) so he can provide some quality minutes at both the 3 and 4 and his contract isn't terrible. Smith is a solid good #2/3 type player that does a bit of everything well.
It also works as Pierce, Allen for Smith, Williams, Hinrich (not that I would do that, but might if Atlanta also threw in a first)