The most popular way to improve the braking power of the E36 is to install four pot caliper brake kits from manufacturers like AP Racing and Racing Dynamics. I have done several Racing Dynamics kits and quite satisfied with the performance. I am going to try AP Racing's kit in about two weeks time, I think. They are developing a 330mm disc kit for my company. The major difference between the two kits will be in firstly disc size (AP is about 20mm bigger), and secondly in the disc material. Racing Dynamics' disc is in one piece, like OE disc whereas the AP disc has an alloy bell housing (centre).
I do not know how much they cost in the States, but the retail price here in Hong Kong is US$1530 and US$2180, with AP being slightly more expensive.

Wilman
GTi Engineering
Hong Kong