If you wanted to convert to the GSR or Si rear disc brakes here's a list of the things you'll need (anybody else correct me if I'm missing something):
Complete GSR/Si Rear trailing arms (discs, calipers, nuts, bolts), brake lines.
You'll also want the GSR/Si proportioning valve and the master cylinder if you don't want to the "flooring" the brake pedal just to slow down. Look on ebay, type in something like "Honda Rear disc brake" and you should find some things you need. (expect around 300 Benjamins). Though if you want to get away for cheaper, you should check your local pic-n-pull.