I'd say Kurt Warner who has one ring (but led two different teams to the Superbowl) is a Hall of Famer for sure, and Romo has a ways to catch up to him. Should Romo win the Superbowl, he'd still be behind Phil Simms who won one and led his team to another before surrendering to his backup. Simms isn't a HOFer, and I don't think Plunkett who outright won two rings is either.
Romo has great passing stats, but would need to contend over the next three or four years to be considered. Even still there will be those who just flat out hate/don't respect the guy who will do their best to block his entry.