Heres a link to a website that does superchargers and turbos.
http://www.koperformance.com/BMW/turbo_ ... /index.htmAs far as everything else goes you could remap the e.c.u and grab a bit of power out of that.
Personally though i feel the best way to get more power would be by selling your car and buying a 323, 325 or bigger. BMW are really good cars mechanically but the more power you start squeezing out of the 1.8 engine the more failures you will find with your engine. BMW designed the 318 engine to put out 110/120 bhp and not 200bhp, the more power you start squeezing out of the engine the more parts you are going to have to replace for bigger, stronger parts and that will start getting very expensive. If you dont replace them you will end up sending chunks of metal tearing out of the engine and be left with a buggered engine and a huge bill for the rebuild.
If you still want the car to be faster, id, instead of getting more power, would put her on a diet and strip every non essential out, spare wheel, tools, sound deadening behind door cards, carpet, every bit of plastic you can see, abs unit. Then if you wanted to put some money into it you could replace the glass with plexi glass, carbon bonnet, carbon boot lid, replace all the seats with light weight bucket seats. There is an endless amount you could and you could end up saving in excess of 200 or 300kilo (same weight as 4 adults). You may not end up with up the same performance as if it had a supercharger but it will certainly run circles around it with its new handling capabilities.
Sorry for the really long reply but thats just my view.