Contrary to popular belief, C++ is quite easy to learn. It does however start to get complicated when you get to pointers, and inheritance and polymorphism. And then gets more complicated yet when you go into SDL (2D) and openGL/DirectX(3D). Stuff written in C++/SDL/openGL/DirectX will run much much faster than something made from DarkBASIC, not sure how it does compared to DarkBASIC pro, but you can bet your butt it's still quite a bit faster.
You might think of learning C++ for use with the dark game SDK, which is very groovy, allows the use of object oriented, inheritance and polymorphism, etc, all combined with the ease of darkbasic. Furthermore, with some knowledge of advanced C++ you can actually cut out a huge chunk of the sdk if for example, you were making a game purely in 2D.
There are many very good tutorials online, for example
http://www.cplusplus.com/doc/tutorial/, and an extremely good book to look at is
http://www.amazon.com/gp/product/1592002056/ref=ase_gamedev/102-5873976-1512134?s=books&v=glance&n=283155&tagActionCode=gamedev which will give you a very solid grounding in the language. Though it does teach you about the same stuff as the tutorial link provided, it does go into more detail, and shows some examples of what you can do with what you've learned.
Hope this helps
Cheers
-J
