I have a dev folder with all my projects. Some of these are on github and some are not. I also use Dropbox (with symlinks) to keep my data synchronised across several computers.
For example if I add something to my Documents folder on one PC I can then see it in the corresponding folder on another PC.
My question is: If I do the same with my dev folder (so the dev folder is synced by Dropbox on both PCs) will it cause problems with my pushing to github?
You don't ever want to mix code versioning strategies. Either all of your code lives in git (which is a good idea), or it all lives in Dropbox (which doesn't give you any history, hence a very bad idea).
When you add a source file to git, you should be forced to push it to Github so it can be pulled at a later date.
I get the feeling that you will run into issues when pushing the code - you'll be adding new files in through one source, but pulling through another - it'd turn into a headache more than a benefit.