purchasing a chip will certainly not turn a 323 into a 325. The 323's have OBD2 emissions controls on them, incorperating much more (and different) hardware. Also, it is illegal in the US to remove emissions control devices on a vehicle or to swap an older motor into a vehicle. If you're honestly looking for more power, the best first place to go would be to purchase a 328i or a 325i, if cost is a factor.
In my opinion, it is never worth trying to build for yourself something that was available from the factory already. It will cost more and be worth less in the end.
You could probably sell your 323 and purchase a 325 with the cash that you got from selling yours. You might even have a couple of bucks left over.