What is better to put in my p4p800 vm 2gb or 4gb???? and run good game with quality ???

Best overclock is to put same manufacture RAM with same frequency in same colors sockets, if you put 4GB it never be 4GB because it will be used anyway.

The more ram you can get in the faster your machine will run especailly with graphics intense operations, like games, video and editing of the later. As important is the speed of your processor. If you have a slow processor, you can't change that, but generally speaking more is best when it comes to RAM