Originally Posted by The Damp Moose
It might have something to do with my having the minimum possible video card (Geforce 5700 FX).
Oblivion uses a huge number of shaders on everything. The FX series of cards are incapable of using more than a couple at once due to some architectual constraints. Could be that when you get hit by a fireball it creates a big heat hazy pixel shaded cloud which pushes your card too far.