I'm in almost the same situation. This spring I will be finishing up my CS degree with a math minor and only few upper division math courses by the time I graduate. I know what you mean about the math GRE (I only got in the 500's) My GPA is pretty good too, but my school is not very good (which is also why I'm not just taking those classes at my current school), so I think that'll hurt me more than my GPA.
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.
If anyone else has anymore advise I would appreciate hearing it as well.
The more I learn the more I realize how little I truly know.
