I found this awhile back and thought it was pretty helpful. It addresses initial investments to get a 350z, Rx-8, S2000, or Impreza ready for drifting.

http://www.jdm-option.com/eng/feature/05_09/report.html

It looks like it's about 3 years old, so there may be some better parts out now. But overall it seems pretty good, imo. From my reading on the rx-8 it seems like you'll never turn it into a powerhouse without serious $$, so there are better ways to go about settings it up for drifting than focusing on power.

In summary, their recommendations are: stiffer suspension, reliability(oil cooler upgrade), and smaller tires(less power required to break traction).

Not sure if this car will ever be a world class drifter, but from the videos I've seen it should be possible to at least have fun. I'm also thinking that dropping weight is another good area to look into(leather seats, sunroof, etc. are all pretty heavy) since it seems pretty easy to do.