In terms of price Vs. preformance, turbo or supercharger, but it wears down your engine much much more and the automatic transmission kinda kills the point of a supercharger because of it's serious loss of power through the torque converter.
For a REALLY instant & cheep increase in preformance, swap differntials for one with smaller gears. Your top speed will be a whole lot less, but you'll acelerate faster. Of course, this is just really a temporary solution and isn't a power increase, just a conversion.
I'm *not* really sure but it *may* cause the AT's computer to be confused since thr RPM's dont match the expected speed. I doubt it though.
In all honesty, it's best to sell it and get a 328 or 330 or something.
Engine swap: Engine block, engine computer & wiring harnasses, new ECU, new transmission, new driveline, new differential (auto transmission E36 diff's have much much smaller gearing than manual transmission E36 diffs) and of course, all of the engine components - water pump, AC compressor, alternator, exhaust (including headers, cat, and muffler) You'll need to beef up the suspinsion a bit to handle the extra engine weight & I'm sure there's a lot more that I didn't list...
To really be cost effective, you need to swap with at least an M engine, V8 (like frank_de_jonge is doing) if you can... BMW engine's have so many electrical components & sensors, it's crazy.
Hopfuelly frank_de_jonge will see this thread and post what he's had to do to the car. If you search the forums for his threads, you'll see a couple threads where he posted pics of his chopped up & very kickass 318