I hope you don't run your car on the street... I was reading the current isssue of "Sport Compact Car",I believe it was, and the raised a VERY good point about roll cages in street cars. They were saying that unless you intend to drive around wearing a helmet all the time, you shouldn't install a roll cage in a street car. If you should get into an accident, the bars car do some major head damage. They also said something about how the roll cages actually interfere with the crumple zones that manufacturers design into their cars. Yes, roll cages look cool, but for a street car, I think would tend to lean a little more towards safety than coolness, IMO.
I had a 14 to 16 point cage in my rally prepared RX7. The car was street legal and we drove it all the time. The cage was a real pain in the ass for street driving. zsprdlin is right that there are too many bars (in most situations) too close to your head. I would not recommend a cage for a street driven car. If you must have one definitely go with fixed back race seats positioned so that your head will not contact the main hoop. There's little you can do about the side bars above the doors except pad them. Use FIA approved padding as its denser than the cheap foam padding most people use.
The Sport and Compact car issue (this months) mentioned above is great. They talk about the ten most popular fact/fiction mods. This includes the benefits and detriments of big brakes, lowering springs, body kits, etc. Some of it is pretty elementary but they get the basic point across. It's definitly worth reading.