Hmm, well, I what I would do is first off, run some kind of texturizer or grain filter on it (film great might be a good start) on a big patch of color or a gradient. Then add brushes and perhaps play with the blending modes, like overlay or screen (if you are unsure how to do this, PM me, I'll be glad to help).
Brush wise, you could try nightmares from Angelic-Trust.net, Splash also from AT.net, Ink from MissM, Splattered from MissM (
http://www.rebel-heart.net/brushes/), soft_plumes which is a default PS7 set, Grunge fettish or fresco from ToastSnatcher (
http://www.toastsnatcher.com/brushes/)...or you can browse around at
http://www.8nero.net/brushes/ which has a veritable TON of brushes, or search at
http://www.xperimentation.com/vbrush/gosearch/index.html, which is a library of brush sites. Hope that helps!