as bushboy, never use more than 100iso unless you don't care about having a picture with more grain than wiltshire.
hi iso's are often offered on low end compacts and that as their lenses are so small (something about f values) and the sensors equally small that not much light falls on them, so in order to get a picture thats not blurred unless your in a sandy bright sunny desert, they use higher iso's than would normally be used.
on the fuji I tend to use 100, as using 64 is OK in good light, but for extended shutter periods can produced excess noise just cos its been open longer.
also, on a digi cam, the iso isn't how reacitve its 'film' would be, instead, just how much the info picked up from the sensors are amplified.
and as most of us should know, amplifying something with a tiny gain like an early cassette, or vinyl for instance, will produce a lot of noise (I say this, but then realise digital tv for example has very little gain
hence its so quiet)