For majority of daylight shots with X5 you will get similar result using any ISO from 100 to 800, but every camera has its native sensivity of sensor, where it produces best possible dynamic range and least noise. At native ISO of sensor, its signal does not have to be gained up or down, so you get cleanest sensor output. That does not have anything to do with overal exposure (as you can change sutter speed and iris to influence exposure as well). So, if you are trying to get widest possible range from whites to blacks with least noise in shadows your best bet is native ISO of sensor, which for X5 happens to be somewhere arround 400. If you gain down 2 stops, to ISO 100, you should get good shots with no noise but with a bit less dynamic range; if you gain up 2 stops to ISO 1600 your result will be a bit more noisy in shadows and range should suffer as well, but if you gain up full 4 stops to ISO 6400 you will see obvious noise for sure... I hope this explains and helps
