Advanced GM isn't really needed, but some topics you may want to learn before next year include: complex numbers, polar coordinates, vectors and kinematics, as these you don't do in Methods 1/2. You may also want to do some work on hyperbola and ellipses. The calculus part of Spesh builds on from Methods 3/4, so a lot of schools encourage students to do Methods 3/4 and Spesh 3/4 at the same time. So, a lot of the stuff you need to know for Spesh 3/4, you learn in Methods 3/4.
B+ average in methods should be sufficient to do spesh but it depends on the difficulty of your tests/SACs. If your at the top of your cohort for Methods 1/2, then you should do quite well in Spesh 3/4. I would say that just doing well in methods 1/2, doesn't mean you'll do well in Methods 3/4 and Spesh 3/4.
The important thing is to try your best and work very hard.