Collision is always a funny topic.
Usally we detect collisins by putting objects into easily comparable shapes. If it were 2 circles it would be just checking if the distance between the 2 center is lower or equal to the sum of their radius.

You can do it simple by assuming that the rectangle is a circle but that would pose problems on faces.

Or you can do it the hard way :

http://stackoverflow.com/questions/4...n-intersection