You can't go wrong with either the A series or the SD series. The SD series is just that much smaller for easier transport in a pocket, the metal body and proprietary battery are about the only big differences between the two.
I got myself caught up in the ISO ratings as well and Fuji's seem to really market themselves as having the best ISO speeds but in real world tests they are no better than any of the other cameras out there.
There is a different style of chip(the name escapes me rigt now) that has much better low light response but it has never made it main stream. It is only available in SLR's and expensive ones at that, only just one brand if I remember right.
