In brief, density is proportional to the number of iterations. Larger values give better quality at a diminishing marginal rate. Filter radius is a measure of the size of the blur applied during anti-aliasing and works in conjunction with oversample, which, traditionally, is the linear multiplier for the image dimension for anti-aliasing purposes. It doesn't quite work like that in Apophysis, but for simplicity's sake, an oversample of 2 means the image is rendered at twice the linear dimension before being reduced - this process (anti-aliasing) reduces jagged edges and some noise.
I'd recommend doing a test render at density = 500, oversample = 2 or 3 and play with the filter radius settings in the Post process dialogue (check the box 'Post process after render') - you'll probably need to save and view the image externally for each change.
You can find out more than you need here: http://ideviant.deviantart.com/art/Rendering-Optimisation-1-73851834
Bear in mind that density was formerly called quality.