Particles and collision detection

I have created a particle system which I will use to create various special effects. I will use it in the creation of explosions, rockets, smoke, trails and other stuff. I have looked at another particle system created with XNA called Mercury and created a similar one. You can add different kind of modifiers to an emitter to simulate things like gravity. Modifiers can also change values of a particle, for example: opacity, color and scale.


I'm also looking into creating better collision detection between players/enemies/bullets. Up to now it has been a small fixed AABB (Axis Aligned Bounding Box) that has been the same size for all models. Now I want to change that and automatically calculate the AABB for a model. When the model rotates or scales the AABB will be recalculated. Later I will change this to use a OBB (Oriented Bounding Box) to rotate with the model (this will result in better collision detection), but I have not yet found a solution for this.