Tough one to figure, the Spurs, while aging, are still the one team that is most intimidating. They have played together for so long they flat out read each other's minds when it matters, I think that is one of their greatest strengths, they all know their roles and they all fill them so well.
If they have a weakness, it is the diminished ability of some of their key bench players, Barry and Horry. They have never seemed to be too concerned with standings, once they are in the playoffs, tha awesome team chemistry pushes them to a very high level of play. They are the one team that scares me whenever we play them. However, their road record has to be a concern, even if it is still fairly early.
Could they add another piece? Sure, who couldn't? But who will fit that role, not Webber, not a team player, although his slow style of play wold be a plus, he is not Spurs material. Stoudemire gives Parker some rest but as has been pointed out already, he is nowhere near the impact player Tony is on this squad.
No big men come to mind that would fit, and besides, until unseated from their throne, why mess with a club that epitomizes the word "team"? I think they stand pat. they are scary enough just the way they are.