What can cause overheathing on my 00 Bmw 323ci if I already replaced all the cooling system parts?

There could be an air pocket in the cooling system still after part replacement. You could have a cooling fan control problem, not activating the fan when needed. You could have an air flow issue from the front through the condenser to the radiator. You could have a breach of the coolant passages of the engine allowing coolant into the combustion chamber, causing overheating. Testing is required to determine issues with any of these areas.