I've taken:
Advanced Calculus (More like Multivar calc II)
Intro To Abstract Algebra
Statistical Methods (Upper level statistics)
Mathematical Statistics I (Stat/Prob theory I)

So, I'm planning on doing the same thing. I'm planning on applying to another school for a second BS program for a year (summer, fall, spring) to get a stronger base in math. Then I'll take a couple basic upper level courses and jump right into first year graduate courses.

Intro to Analysis (upper level)
Linear Algebra (upper level)
Math Analysis I-II (first year grad)
Linear Algebra (first year grad)
Topology (first year grad)
Intro to Abstract Algebra I-II (first year grad)

I've been told the four most important courses to go into a Ph.D. program with are Analysis, Algebra, Linear Algebra, and Topology. In answer to your question post-bac programs usually just involve going to a school as a non-degree student and taking courses, or apply as a second BS student and taking courses (but not finishing a second degree) until you feel you've taken enough courses, so really there isn't a difference from holding off on graduating or doing post-bac work, unless you want to change schools. My suggestion though is take at least a couple of graduate level courses if at all possible, while not required it looks very good if you can show you are capable of graduate level work.