I have a dual-core 2.3 GHZ Powermac G5 it can take up to 16 GB of RAM

How much Ram do you think I would actually use? how much should I put in?

I mainly use this computer to run Logic Express and Soon Logic Studio.

I also play Doom 3 and Quake 4 but not as much these days.

so ho much RAM should I through in?