Current isn't really "pushed" into circuits. Circuits "draw" power.
The regulator will have a certain amount of power delivery capability. If you try to draw more than that, one of three things will happen based on what the regulator is:
1) The regulator will drop the voltage until less power is drawn
2) The regulator will go into thermal shutdown
3) The regulator will overheat and be destroyed
The reason to put a PTC in a circuit is to protect the power delivery part (such as the regulator) from overheating, as well as providing a "fail safe" in the case of accidental shorts or failures somewhere in the circuit, perhaps preventing various components on the power path from overheating and failing.
The PTC you describe is constructed such that, if you draw no more than 500 mA, it will never "trip." Meanwhile, it guarantees that if you draw 900 mA, it will "trip." (This is likely in 25C ambient temperature -- derate if the air is hotter or the PTC is not mounted in free air.) When "tripping," it will dramatically increase its resistance, thus dropping a lot of voltage, thus reducing (but not totally eliminating) the current through the circuit.
Now, if the PTC is guaranteed to trip at 900 mA, but the regulator is rated for 800 mA, then there is some chance that the circuit could draw, say, 850 mA, which might damage the regulator, but the PTC may not trip. Thus, the conservative design would be to upgrade the regulator to one rated for 1A -- that way, you know the PTC is guaranteed to protect the regulator under all circumstances.
Note that the PTC is a resistor. There will be some voltage drop across it. If you put it after the regulator, then the actual voltage seen by your circuit will sag a little bit as it draws more current. This may cause instability for the microcontroller (assuming that's what you're driving.) As you're using a linear regulator, the current into the regulator is the same as the current out of the regulator, so the right place to put the PTC is before the regulator. A voltage drop there will just mean the regulator has to burn off slightly less power to regulate the voltage, so it won't affect the circuit much except slightly raising the "recommended minimum voltage" input to your device. For example, if you use a 1.2V drop-out regulator, and you expect the PTC to drop 1.5V at 500 mA, and you want 5V out of the regulator, your minimum input voltage will be (5+1.2+1.5) = 7.7V.