The other comments have covered you on the probabilities, however if you look at this pragmatically then you can get a definite answer for yourself.
You said yourself that you are going to be hashing your sequential IDs. It would be easy to code up a test case. Iterate through ~100,000,000 ids and check for collisions. That would not take long to do. On the other hand you might run out of memory quarter of the way through.
