Your problem is more that you're imagining something than describing it.  People have "an idea" of what volumetric light looks like, but what it actually looks like in any given setting is very dependant on the particulars. With these kinds of volume effects, you have to be very precise about particle size . . . the effect that produces a good volumetric fog won't necessarily give you a good dust cloud.

In 3D applications, the general term for this is "participating media" (meaning that the "empty space" that the light travels through actually has some optical properties) or "volumetric lighting". Google "Vray volumetric lighting" or "mental ray volumetric lighting" and you'll find a lot discussion. You'll find a thread discussing dust in volume lights in Blender here: http://blenderartists.org/forum/showthread.php?t=164715&page=1

In 2D applications, the effect can be faked at much lower compute cost with particle systems, these work well for the bright discrete dust flakes that you seem be trying to achieve. In Photoshop, I think I'd just make a brush with a bunch of fleck samples, assign a layer, and start painting.

The big thing is that volumetric lighting is usually very contrasty-- a shaft of sunlight coming into a dark basement, for example-- and things reflecting in the beam of light shold be brighter than you might think.