If it were me, I'd give it an "internal" deadline - meaning, pick a date that things need to change by and then make the decision to give your boss that ultimatum. In the mean time, I'd be evaluating new DBA opportunities to have possibilities in the works...if you're a DBA (and that's your passion) you're not going to be happy in a position like this for very long and it's tough having to drag pre-existing baggage up the hill all the time...

You could also take another approach. Find a new angle. Find something existing business process that's currently in place that takes up significant company resources and find a way to drastically improve it using SQL Server. Present your work-of-art to the powers that be with the slight that "if given the opportunity, these are the kinds of things we can do here with SQL server" and see if they bite!