To be honest, simple C++ knowledge is enough to start with DarkGDK. Sure, more advanced certainly helps as you gain more general programming theory and knowledge, but to get into it and actually start making stuff, all the basic C++ is fine.
As to your question about SDL vs. DarkGDK, I would say that DarkGDK is much easier, especially for a beginner. Firstly, it ships with Visual C++ Express, so there is no compilation/installation problems. Secondly, much of the required stuff for SDL, like windows and other basic things, are handled internally in DarkGDK. Finally, if you ever programmed with DarkBASIC Pro, then you will find DarkGDK a breeze.
Cheers.
Tux is my guildmaster.