I think it really has to do with where your heart is. And that goes for more than just games like this.
I have never played magic but I often play video games with monsters, demons, etc in them. I am able to seperate myself from the game and enjoy it for its plot, intrigue, and mechanics. I defeat the game and put it away.
I do know some people though to go waaay to far. They start idolizing the game/movie/book/etc which can then transfer to worship of the creators and even complete disillusionment. At some point during that process the whole thing should have been stopped.
This applies in many walks of life. A good friend of mine will not touch alcohol. Not because he thinks it is wrong in any way but just because he knows that his personality cannot handle it.
I was a pretty bad video game addict when I was growing up. I still play video games but I severly restrict myself to only 1 game at a time and in moderation. I know that if I go out and buy every video game I want that I will waste my life away rather then spend time with my family or do any of the other things I want in life.
I think that should be the barometer. IF you look at a magic card and see a goblin picture with a bunch of stats and nothing more then I don't see that there is really any problem. If you see the same card and are totally intrigued by its implication in your world then maybe you might not be able to handle the connection between the game and reality.
Listen to your heart. Ask God. Proceed accordingly.
Organizations worth supporting:
Electronic Frontier Foundation | Defending your rights in the digital world (Protect Privacy and Security)
Home | American Civil Liberties Union (Protect Civil Rights)
AAUP (Protect Higher Learning)