All and yes.
In one sentance -> No board made lets you mix SDRAM and DDRRAM.
In what type of applications can we see the performance differences between the rams which are at different frequencies? (For example: sdram 133MHz and ddr 333MHz)
For example; will 3D games (like CS and Hitman) will run fine at ddr 333MHz and when trying at sdram 133MHz, will it slow down because of ram?