In my opinion id say that is a bad idea, but you know what they say about opinions.
Your 318, first of all, is a hell of allot lighter than a 6 cylinder 325, and as we all know, there is allot more to cars than straight line speed. Lighter weight improves cornering, braking and acceleration, and with a decent amount of work, your 318 could be out there playing with the 6 cylinder boys on the straights aswell.
Can i ask what will you be paying for the change from the 318 to the 325?
Eg: the price of the new car minus the price you will get for the old car.
If you take this money and invest it in performance modifications to your 318, you will end up with a far superior car.
I would suggest suspension and brakes first, then move onto engine work.
if it is at all possible to convert the 318i to run on carbs then it would be fantastic.
imagine a 318 on dual weber carbs, a ported head and a lairy cam, sideways through a country road......that is a much better idea than a bog standard 325.
You dont want to hijack swiss 318is's thread, so why not start a seperate thead to discuss your ideas?
Good luck with your car, and i hope you make the right decision.
Anyway, back to the Swiss maddness:D:D:D