About SDRAM clock ! ?

Running the processor at a different clock speed than its program memory will add domain crossing delays that will make your processor run much slower, worse than running everything at 100MHz. Moreover, SDRAM are not suitable for the multiple clock domain architecture without having complicated memory controllers capable of burst access.

If you have a bandwidth hungry peripheral like a video controller, a better solution is to play with the bus priorities in SOPC Builder and if that is not enough, consider adding a dedicated video memory chip.