I kind of feel like magic in Christian games should play the part it plays in reality. People turn to Satan, knowingly or unknowingly, in order to do things that they should not be able to do (such as psychics, telekinetics/levitation, and just spells in general). It should be shown as evil, though the person could be redeemed. The person may not have even asked for this, it may have been forced on them - like that girl that Paul and Silas went to jail for expelling her demon. It was a common practice in their day to force a slave into a ritual that would invite a demon into the her (the oracles) so she could predict the future. This would provide a source of income for the slave owner while the girl is tormented - truly evil.
Maybe magic can fall into several categories, all of them evil but not all being voluntary.
As for good 'powers' that a Christian or Biblical hero can use, prayer is obvious, as is God's own action. But these are mostly passive things, gamers want to take action. As Christians we have been given the ability to cast out demons and do miraculous things in the name of Christ, as someone mentioned earlier. That could be used. Also, if the character in play is an angel it could get much more fantastic and give the player a more 'magical' person to play.