The bottom line is, Smith/Williams appears to work, by the numbers, for any of the following:
KG straight up
KG + one of Stiemsma/Johnson/Bradley/Moore
one of Allen/Pierce + one of O'Neal/Bass
So it's not like we don't have packages to offer Atlanta depending on what they want. But whether they're open to trading these two is a different issue altogether.
For what it's worth, Smith has made some eyebrow-raising comments lately too, such as that he doesn't feel he has any increased responsibilities with Horford out.
Would any trade in which we trade for Smith/Williams be accompanied by the amnesty of Paul Pierce? It's the only way this team would have a shot (however slim) at cap space before 2014-15 (when Williams/Pierce's contracts expire). Smith's contract expires one year earlier but it won't get you much cap space assuming Danny signs a few other complementary pieces. That's the main reason I have a tough time seeing a trade for Williams while Pierce is still on the roster-- you'd have $22.5m tied up in the SF position for the next three years.
PS: Marvin Williams is comparable to Jeff Green with a worse jump shot but better defense and rebounding. They're both 25, big SFs who aren't particularly great at anything but do everything OK. Williams has a better career TS%, rebounding percentage, and turns the ball over less, but other than that they're near-identical players statistically. Again, he's overpaid, but he's a useful player.
Josh Smith, on the other hand, is a borderline-elite rebounder and shot-blocker who plays good team defense. His jump shot is shaky but he has elite athleticism and a decent post game. He's only 26 and on a reasonable contract. Again, one of the few available pieces I'm willing to build a team around in the post-big-3 era.