Things get real interesting around the "ultimate reaches" of power. The War Powers Resolution of 1973 was vetoed by Nixon, but put into law by a Congressional supermajority. The Constitutionality of that law has not been put to the test, but at least Dick Cheney (and many others) hold that it represents an unconstitutional encroachment by Congress. So far the encroachment has been tolerable, but that doesn't mean it will always be followed by a President.
The question of whether or not FISA provides an unconstitutional exercise of legislative power over presidential power is likewise not amenable to a "neat" solution. If the President persists in action that Congress finds objectionable, the House has the power of impeachment. This is a political power at its core, with ultimate exercise by "we the people" at the voting booth.
If the War Powers Act is unconstitutional, then it would almost have to follow that the Posse Comitatus Act is likewise unconstitutional, because both are acts of Congress that purport to limit the President's discretion as commander-in-chief.