If the marriage was bona fide, nothing. You will need to seek waiver of join removal condition filing requirement.
The information contained in this answer is provided for informational purposes only, and should not be construed as legal advice on any subject matter,not should it be viewed as establishing an attorney client relationship of any kind.
It's fraud to apply for a green card through marriage when you have no intent of staying married to your spouse. You're right to be concerned. The process is more complex than what your girlfriend thinks, and it's serious business. Do not abuse the process, you might have serious consequences for doing it.
On the other hand, if you do decide to get married in good faith, once you file the application it would take about 3 months to receive a travel authorization. I encourage you to consult in person with an experience immigration attorney who can review your immigration history and assess your options. Good luck.
[This answer is for general purposes only; it does not constitute advice and does not establish an attorney-client relationship.]
You should be concerned about what happens if a divorce occurs before you acquire permanent residency. I have some clients who have found themselves in sucha situation. Your best bet is to get an immigration attorney who can help you document the bona fides of the marriage so that you can have evidence sufficient enough to apply for a waiver if the marriage does not survive. By all means, make sure that you are getting married because you intend to be with thi woman and not for a green card. All the best.