Darkbasic is a good starting place for making 3d games. However it does have its limits. Once you hit them then upgrade to something like leadwecks.
I have been using darkbasic for a long time now and it easy to pick up and use. The good thing is its a great as an idea tester for later on as it can handle most things. For me games programming is a hobby not a job. I learn new things every day that will help me in the long term like sharders and AI.
Your C++ will be helpful in that you know how to set out a program and you can write dll files for darkbasic.
As for the massively multiplayer online game it will take a long time and darkbasic can be used as a front end but your need to code the server part in something like C++. Just boils down to speed and that your want the server to do server things not render graphics and the like.
Also don't jump in the deep end start small and build it up to it. E.g. Build a rpg and then build a massively multiplayer online game the good thing is your have most of the code already done.
I'll add something later on.