Unless your heavy multitasking involves running many virtual machines simultaneously while gaming, or trying to edit HD video while gaming. or you want to have a 20+ GB RAMDisk, I doubt you're going to need 32+GB of RAM. If you really want to give yourself some extra headroom, go for 16GB, no more than that unless you know you have an application that can eat up huge amounts of RAM on its own.
As for speed, it really doesn't matter, in the real world there is no discernible benefit from having RAM faster than 1600MHz. The only exception is if you are trying to game with AMD's integrated graphics on their APUs, then having faster RAM will show some benefit. Aside from that one use case, going for the ultra fast RAM is pointless unless you are after bragging rights for getting the highest scores in memory benchmarks.