But seriously, there won't be anything that gives you a super bang for buck for a 318. Any sort of 'stroker' motor build type deal will cost way more than the aggravation is really worth. Do a 6 cyl swap.
Software and diff gears will be your biggest gains, other then that it's all small gains. The e36 is alot bigger of a car, so in comparison to upgrading an e30 318i, it's a differet ball game, and having a 95 318i, the software would be about as far as I go with the car.