A friend told me that if I purchased a recent copy of Mario Kart DS it would have 3D effects added if I played it on a 3DS. I have an older version which doesn't work, but my friend informed that it had to be a recent version of the game. Is this true? Do newer DS games get 3D effects if played on a 3DS?
There will be no 3D effect with a regular DS game, as this IGN article states:
Standard DS games will not have the ability to be displayed in stereoscopic 3D on the 3DS. DS games will...only be displayed in standard 2D.
To further the point, on this comparison chart from Nintendo, it states at the bottom that:
Only Nintendo 3DS software can be played in 3D. Nintendo DS and DSiWare software will be displayed in 2D on a Nintendo 3DS system.
The only way you will be able to play Mario Kart in 3D is to wait for Mario Kart 7 to be released in December.