I totally agree with nick here. The player already has chips in for the call, so throwing in another 500 chip would constitute a raise because removing it still leaves enough for a raise. Rule #39 Baby! I vote Raise..
I'm surprised that everyone is coming to the right conclusion with the wrong reasoning, including you Jaspertoo!
I totally agree with nick here.
Ok this part surprises me.
The player already has chips in for the call, so throwing in another 500 chip would constitute a raise
YES (you could've stopped right here!)
because removing it still leaves enough for a raise.
NO (even though what you said is true)
because removing it leaves the original 500 chip which is enough to COVER a call.
YES (fixed)
and see first quote followed by YES 
I miss all these reindeer games 
It's been awhile since I have been on the boards. Lost my Mom over Thanksgiving and been kinda busy since.
I don't disagree with Nick that much, do I??

lol...
However, McGee, you say NO to the part where I say removing the 500 still leaves enough for the raise. It does after all leave enough for a raise, (100/200 - 500-200=300 Raise). But I imagine that is not exactly the point you are making by disagreeing there. Your corrected statement is quite correct, BTW!!!!! Thanks! (and more to the point)
But, really, the one chip rule here does not apply, I don't think. I believe that since he already has what he needs for the call, throwing in ANY chip would be the same as declaring a raise. And that raise amount would be for whatever total chip count he had in there at the end, under these circumstances.
I always love the argument players have with us when they have chips in for the blind and they toss chips in thinking they are helping by giving "change" chips. They always lose sight of the fact that the chips in for the blinds are no longer their chips, but the pots. different thread I suppose......