Okay... I could be wrong... but... isn't the FSB just a connection from the CPU to the RAM? If I'm right then your RAM speed will play a big role in that performance percentage... If your RAM (in effective MHz... 2 sticks dual channel for example) is slower than the 533 MHz FSB then you will see no performance gain going to 800... if it's faster than 533 then you will have that extra head room.