Does having a GPA above 3.3 guarantee admission to the accelerated BS/MS programs? 

No. The GPA threshold is a necessary but not sufficient condition for admission to the accelerated BS/MS programs. The admissions committee will typically focus on your grades in MATH and CS courses while evaluating your application.

I am planning to take two CS 500 level courses and three CS 400 level courses in my last semester as an undergrad at Mason. Is this a good plan?

Probably not!

Students should be aware that the workload associated with a CS 500-level graduate course is higher than that of its counterpart at the 400-level. Consequently when deciding on your plan of study you should meet with your academic advisor.  In addition, every student admitted to the BS/MS accelerated program is required to meet with the director of the MS program before his/her first semester in the program.