Insulation, Radiant and Vapor Barriers - insulation for 2x4 roof

I have a 1910 bungalow with a "finished" attic bedroom - I just tore off all of this ugly 1/4" fake wood paneling to replace with sheetrock and learned..
1) the previous owners replaced the roof which must have been shake with composite and the insulation (pink R-11) is convered in dust and small wood roof fragments. Other than being dirty most of it looks in decent condition but I don't know how old it is.
2) The roof joists are actually 2x4

I'm trying to decide if I should replace the insulation and I am concerned with the ventilation because it's such a tight space. What should I really be concerned about and what should I look for? Are there other options I should consider? The room gets pretty warm in summer but i just put in a couple openable skylights. In winter it seems warm enough - i don't think my heat bill is unusually high and I live in Seattle - fairly temperate but rainy. Thanks!!!

The dept. of energy recommends that attic be insulated up to R-49, which is obviously impossible with such small rafters. I'd suggest that you add additional framing (like a 2x2 or 2x3) onto the bottom of your rafters so that you have additional depth. then consider having someone come in and spray the ceiling with icynene insulation. A closed roof system will give you the best r-value.