I installed ddr400 (there are 2 slots for it too) while there are already two sticks of ddr2 533. It doesn't work and the computer doesn't start, do I need to change the fsb of the whole system for it to work (to 400)?
You should check your motherboard manual, but to my knowledge there are no motherboards that will allow you to use DDR and DDR2 simultaneously. If I'm not mistaken you have to stick with one type of RAM on the boards that allow use of both kinds. They make those types of boards so that people who can't afford to upgrade their whole system can upgrade the processor and then swap out their old DDR for newer DDR2 later.