Bus speed determines several different things. First, bus speed will determine the speed of the memory which you use. A 333mhz FSB will only be able to utilize a memory bandwidth of 333mhz, while a FSB of 400mhz will be able to utilize a bandwidth of 400mhz. Obviously faster is better.
Also, the FSB determines how far a CPU can overclock without adjusting the multiplier. If you don't plan on overclocking, this shouldn't be an issue. The CPU speed is determined by two things, the FSB speed and the multiplier, for example, a 2Ghz proccesor may have a FSB of 200mhz, and a multiplier of 10. 200 x 10 = 2000
By slowly increasing the FSB speed, you can slowly overclock a CPU. Obviously a higher FSB speed will be able to overclock higher.
Also, the FSB determines how far a CPU can overclock without adjusting the multiplier. If you don't plan on overclocking, this shouldn't be an issue. The CPU speed is determined by two things, the FSB speed and the multiplier, for example, a 2Ghz proccesor may have a FSB of 200mhz, and a multiplier of 10. 200 x 10 = 2000
By slowly increasing the FSB speed, you can slowly overclock a CPU. Obviously a higher FSB speed will be able to overclock higher.