I don't pretend to be a true salary cap expert, and there are people on SR that know far more than I do, but HoopsWorld gives a pretty good summary I think: San Antonio Spurs Team Salary | HOOPSWORLD | Basketball News & NBA Rumors
Next year's salary cap should be roughly $70 million, of which we're already on the hook for roughly $38 million assuming Diaw and Mills exercise their options (definitely a safe bet on Mills, and almost certain for Diaw as well). That gives us roughly $32 million to work with, with Ginobili, Blair, Bonner, Neal, and Splitter unsigned.
I see Ginobili coming back at maximum $7 million, though I can't imagine more than that (remember, he was the highest-paid Spur this year). Blair's gone, so I see Bonner coming back at $4 million (his non-guaranteed offer, not that I really know what that entails). Neal's qualifying offer is $1.1 million, and I imagine he'll come back at that rate.
That'd leave ~$20 million. Priority #1 is re-signing Splitter, I think, and with the market for big men what it is, I think it'll take around $10 million. I don't think Splitter is good enough to warrant $10 million, but I think the Spurs are $10 million better with him than without him, so I foresee him staying while chowing down on that salary cap room. That'd leave us $10 million to work with while retaining all the major cogs, good enough to bring in a mid-level free agent if we want.
Who could we get for around $10 million? With Blair gone and all our shooters still around I imagine we'd target another bigger body, especially after our Finals victory over the Heat in six games reveals the importance of size in an increasingly small NBA. Big men are hard to come by, though, so the more likely result might be a strong perimeter defender (Tony Allen, anyone?) or another solid spot-up shooter (Redick, perhaps).
I could be totally misrepresenting the cap, though, and I also am completely in the dark about the function of the various exceptions and Bird rights and what have you -- I'm speaking as if it were a hard, straightforward cap, but that's not really the case.