I'll echo what dxq says about the a100, but will add that the a200 is better at higher ISO than the a100, the a300 is better at higher ISO than the a200, and so on, and so on.
I have the a300 and find I can bump up the ISO to 800 and not notice any noise in the image, but when I go to 1600 it is noticeable, but not too distracting. The other good thing (again, as dxq mentioned) is that you can get Minolta lenses that fit the Alpha series. I have 2 myself (50mm f1.7 and an 85mm f1.4) This means you can get good lenses cheaper.
All this is moot if you want to get a Canon as Sony/Minolta are the only lenses you can swap mounting.